Abstract: A locking device is integrated in a coupling fixed on a ram cylinder for feeding the cylinder with a first fluid. The circulation of that fluid is controlled by a second fluid. The device has (1) a cylindrical member with a tightening head at a first end, a second threaded end to engage in the cylinder, and an intermediate portion between the two ends. The intermediate portion has an axial outlet passage to the cylinder, and a lateral opening terminating at the passage. The device has a coupling head with a through bore, and an internal passage for the first fluid. The bore is shaped to receive the intermediate portion and to couple the opening to the feed passage. The device has movable internal members to modify circulation of the first fluid by moving under the combined effects of a resilient element and the pressure of a second fluid applied to an external locking inlet.

Abstract: A load check valve is normally employed in association with a double-acting hydraulic cylinder, such as the type used to raise and lower the boom of a construction vehicle. Conventional check valves of the poppet type are difficult to modulate whereas check valves of the spool type generally do not insure leakage control at an acceptable rate during normal operation. The spool-type load check valve (20) of this invention, used in conjunction with additional check valves (60,61), overcomes the latter problem by providing a series of interconnecting passages and chambers (36) for communicating equalized fluid pressure from a cylinder (11) to either end of the spool (27) thereof to provide for minimal leakage.

Abstract: A fluid pressure operable jacking tool having oppositely disposed locking arms. With the draft key and coupler removed, the locking arms of the jacking tool, when installed, biasingly engage the coupler cross key slots formed in the draft gear yoke, so that a fluid pressure powered ram of the jacking tool abuts against the draft gear follower. A fluid pressure line is connected to the jacking tool and, when provided with fluid pressure, causes the jacking tool ram head to compress the draft gear to allow removal of the draft gear and jacking tool from the car center sill together. Once removed, pressure is released from the jacking tool to permit separation thereof from the draft gear. If the draft gear is to be installed, a reverse procedure would be employed.

Abstract: A hydraulic control system with a pipeline antiburst safety device for a double acting drive cylinder, more particularly for adjusting and securing the position of a driven guide roller in the strand guide of a continuous casting plant which exerts an operationally conditioned reaction force, applied by the strand, on the piston of the drive cylinder.

Abstract: A control arrangement for lifting and lowering a hydraulically operated implement comprises a precontrolled reversing valve and a precontrolled stop block arranged in the fluid pressure stream leading from a source of fluid pressure over the reversing valve to the compartment of a cylinder and piston unit connected to the implement for lifting the latter during feeding of pressure fluid into the compartment and for lowering the implement during discharge of pressure fluid from the compartment. The stop block comprises a releasing piston cooperating with a stop valve of the stop block to alternatingly connect the compartment to a return conduit leading to a tank to discharge pressure fluid from the compartment or to the reversing valve to feed pressure fluid into the compartment. A pressure valve arranged in the releasing piston controls a connection leading from the reversing valve to a damping chamber at one end of the releasing piston which is additionally influenced by the movement of the latter.

Abstract: A fluid control system for selectively holding a work implement, such as a blade in a motor grader or the like, in a fixed position in a first operation mode, and for selectively providing a shock absorbing system (70,72) for the implement in a second operating mode. In the fixed position mode, lock valves (36,38,68) are operative to provide positive locking for an implement actuator (24,26), while in the shock absorbing mode, longstem lock valves (82,84) are operable to maintain one check valve (102) of a dual valve structure (96,98) unseated at all times. Each unseated check valve (102) provides a connection of one end of an implement actuator (24,26) to vent to insure that the shock absorbing system (70,72) maintains a full shock absorbing capacity.

Abstract: A regulating device for a working cylinder of an agricultural vehicle comprises a directional multiway control valve which controls the flow of a working fluid to the cylinder via a switch valve which is normally biased into its closing position and held in its open position by a pressure difference applied thereto from a throttle arranged in a main control conduit connected to a control fluid pump. One end of the spool of the directional multiway control valve is spring biased into a lowering position and its other end is controlled by a setting cylinder which in turn is controlled by an adjustment pressure having an actual pressure component and a desired pressure component. Means for setting the adjustment pressure includes an adjustable pressure reducing valve connected in series in the main control conduit with an adjustable pressure relief valve, the adjustment means of the latter being controlled by an actual power measuring unit on the vehicle.

Abstract: An absolute-control numerical hydraulic actuating device with display of the output position, comprising a hydraulic jack, a distributor formed with orifices and a selector connected to the orifices in order to make the required hydraulic connections for hydraulic locking in the desired output position.According to the invention, the orifices of the distributor can be selectively connected by the selector to the active chamber of the jack in order hydraulically to lock the output positions in accordance with numerical orders supplied to the selector, and additional intermediate locking positions are created by the provision of additional rows of orifices which can be selectively connected to the high-pressure source, to the low-pressure source, or to the active chamber of the jack through a further selector.

Abstract: A control arrangement for a hydraulically operated load lifting and lowering device includes precontrolled first valve means controlling flow of fluid from a source under pressure to a return conduit leading to a tank or to a working conduit leading to the device and a closer block in the working conduit, including a piston controlling flow of pressure fluid from the working conduit into the device, respectively flow of pressure fluid out of the device into the return conduit downstream of the piston. A throttle is arranged in the return conduit to produce upstream of the throttle a pressure acting on the piston but not on the slide of the first valve means, whereby without increasing the neutral fluid circulation pressure a uniform lowering of the load independent of the magnitude thereof may be obtained.

Abstract: PCT No. PCT/US79/00859 Sec. 371 Date Oct. 15, 1979 Sec. 102(e) Date Oct. 15, 1979 PCT Filed Oct. 15, 1979 PCT Pub. No. WO81/01034 PCT Pub. Date Apr. 16, 1981In prior art hydraulic systems, and in particular in hydraulic systems having a cylinder means for elevating and lowering a load, there exists the possibility that the hydraulic line from a control valve to the cylinder means could rupture, allowing the load to drop before the operator could react. The present invention solves this problem by providing a blocker valve (100) in the vent line (69) of a check valve (33,34) which check valve is interposed in the main line 27 between a cylinder means (12) and a control valve (21). The blocker valve (100) is responsive to the back pressure in the main line (27) from the check valve (33,34) to the control valve (27).

Abstract: A protective fluid system having a fluid pump and a fluid control valve and a fluid protective valve and a fluid motor all connected together. The protective valve is responsive to a loss of pressure in a fluid line, such as when the line breaks, and the valve then closes to avoid the reaction of the fluid motor which may be a powered cylinder assembly under the influence of a load, and thus the load does not fall or otherwise move when the pressure is lost in the cylinder assembly.

Abstract: An automatic hydraulic shut-off system for use with a hydraulic valve and a work apparatus which is retained under hydraulic pressure even when the hydraulic line is broken inadvertently. A lock valve is connected in the line and senses the loss of hydraulic pressure, and the valve is actuated in response to the pressure drop and retains the work apparatus in a pressurized condition to thereby avoid undesired movement in response to the loss of pressure.

Abstract: An automatic hydraulic shut-off system for use with a hydraulic valve and a work apparatus which is retained under hydraulic pressure even when the hydraulic line is broken inadvertently. A lock valve is connected in the line and senses the loss of hydraulic pressure, and the valve is actuated in response to the pressure drop and retains the work apparatus in a pressurized condition to thereby avoid undesired movement in response to the loss of pressure.

Abstract: A manually operable position lock for a hydraulic actuator (10) adding a cylinder (20) and a piston rod (22) with a stop finger (61) for actuating a push rod (60) to close a valve (32) and prevent fluid egress from the cylinder. The lock includes a pivot bracket (110) carried by a push rod, a control lever (111) pivoted to the pivot bracket, a link (114) pivoted to the control lever and slidably receiving the push rod, and a helical spring (120) carried by the push rod and engaging one end of the link.

Abstract: A hydraulic valve device for controlling a double action, piston-type servomotor adaptable for steering a self-propelled agricultural vehicle and including a pair of identical interconnected valve groups, with each group connected to an opposite side of the servomotor and further connected to a source of pressurized fluid and an exhaust outlet, whereby selective actuation of the hydraulic device creates a pressure difference in a servomotor forcing the piston to move in a predetermined direction.

Abstract: A control arrangement for a hydraulic power consumer particularly in agricultural machines, has a pressure medium reservoir which is connected with a member for supplying pressure medium from the reservoir to the consumer to thereby move the consumer in one direction. The consumer is also connected with a member for withdrawing pressure medium from the consumer to thereby move the latter in an opposite direction. The arrangement further includes a single valve unit which is operative for both communicating and discommunicating the consumer with the supplying member and with the withdrawing member to thereby alternately move the consumer in the respective directions.

Abstract: A control apparatus for a hydraulic power consumer, particularly in agricultural machines, has a pressure medium reservoir which is connected with a member for supplying pressure medium from the reservoir to the user to thereby move the latter from a stationary position. This member is subject to pressure oscillations which result from a pressure difference which is created in this chamber on displacing of the consumer from the stationary or neutral position. In order to eliminate undesirable oscillations of the user on displacement of the latter from the stationary position the arrangement is provided with a member for damping these pressure oscillations.

Abstract: The present invention relates to a hydraulic brake booster and a shuttle valve which is operable to limit the pressure communicated to a pressure chamber within the hydraulic brake booster. The shuttle valve is disposed within a control valve which is operator-actuated to communicate pressurized fluid to the pressure chamber so as to provide a power assist to braking via a piston responsive to pressurized fluid in the pressure chamber. The shuttle valve is exposed to the pressure chamber and movable relative to the control valve when the pressure in the pressure chamber is above a predetermined value in order to terminate or decrease communication of pressurized fluid to the pressure chamber. At the same time the shuttle valve cooperates with the control valve to reopen communication to an outlet as the control valve operates to close communication thereto during braking.

Abstract: A hydraulic system for raising and lowering a load by means of a fluid motor includes a counterbalance valve which controls the flow of fluid to and from the motor. The counterbalance valve has a pilot operated metering orifice which passes fluid from the motor when the load is lowered and a compensator valve which maintains a fixed pressure differential across the metering orifice. This results in a fixed flow of fluid through the metering orifice when the load is lowered.

Abstract: A stroke-adjusting drive mechanism in which the stroke of a reciprocating member, such as a piston, is adjustable for use in machines such as hydraulic presses and press brakes is disclosed. The apparatus includes a cylinder and piston with a piston rod member which projects outwardly from the cylinder. A restricting valve means is arranged between a chamber of the cylinder defined by the piston member and a hydraulic passage connected with a hydraulic pump. An adjusting member is arranged in the piston rod member so as to be adjustably positioned along the axis thereof and a regulating valve is provided for restricting the flow of hydraulic fluid from the first chamber in response to changes of pressure and the hydraulic passage. By means of the apparatus of the present invention, the stroke length of the piston member can be easily and precisely adjusted as well as stopped at a desired position.

Abstract: In a hydraulic circuit (10) fluid is passable from a priority valve (28) through control and lockout valves (30,32) to, for example, a suspension system (27) of a work vehicle (20) or control cylinders of a steering assembly. The lockout valve (32) provides for controllably operating or locking out, for example, the operation of the suspension system (27) where desired. The control valve (30) controllably passes fluid to the suspension system (27), for example, based upon demand created by operation of the system (27). The control valve (30) also delivers a signal to the priority valve (28) for maintaining priority of fluid flow from the priority valve (28) to the suspension system (27). In the circuit (10), the signal is automatically, controllably blocked in response to locking out the operation of the suspension system (27) for preventing the priority valve (28) from maintaining priority fluid flow to the non-operative suspension system (27).

Abstract: A hydraulic system including a hydraulic pump, a hydraulic motor having at least one port and a hydraulic reservoir. A main spool valve is interposed between the motor and the pump and reservoir and has a spool shiftable between positions connecting the pump to the port, connecting the reservoir to the port, and blocking fluid flow to or from the port. A single combination check and flow control valve is interposed between the main spool valve and the port. A fluid responsive surface opens the combination valve to permit relatively free flow of fluid therethrough when the spool is in the first mentioned position thereof. A pressure responsive surface and a control valve provides for partial opening of the combination valve to meter fluid flow therethrough when the spool is in the second position thereof. A spring is operative to close the combination valve to prevent fluid flow therethrough when the spool is in the last mentioned position thereof.

Abstract: A hydraulic motor and control system therefor which includes a pump for delivering fluid to a control valve, the control valve having a Float position which connects a first end of the motor to a sump, a First position for applying fluid from the pump to power the motor in a first direction by delivering fluid to the first end of the motor via a flow path and a Second position in which the motor moves in a second and opposite direction. A conduit connects the control valve to the sump. A check valve is provided in the flow path which always allows flow therethrough towards the first end of the motor and normally blocks reverse flow therethrough and also a mechanism for opening the check valve to allow reverse flow therethrough responsive to operation of the control valve in the Float position and in the Second position.

Abstract: A hydraulic motor and control system which comprises a pump delivering fluid to a control valve. The control valve has a Float position which connects a first end of the motor to sump, a First position for applying fluid from the pump to power the motor in a first direction by delivering fluid to the motor's first end via a flow path and a Second position in which the motor moves in a second and opposite direction. A conduit connects the control valve to the sump. A check valve is provided in the flow path which always allows flow therethrough towards the motor's first end and normally blocks reverse flow therethrough. A mechanism opens the check valve to permit reverse flow therethrough responsive to operation of the control valve in the Float position and in the Second position.

Abstract: An arrangement for controlling a hydraulic motor having a movable member with opposite faces to be impinged by pressure fluid, in which first valve means serve to direct pressure fluid against a respective one of the opposite faces to thus control the direction of movement of the movable member, and in which second valve means connected in a hydraulic circuit with the first valve means serve to control the speed of the motor in connection with control means actuated by the movable member for controlling the change of the speed dependent on the position of this member.

Abstract: A fluid source for delivering fluid to a control valve having Float, First direction and Second direction positions. A piston-bore check valve allows flow to the motor and normally blocks reverse flow, the piston having its first end biased against a seat. A control chamber communicates with the piston's second end. Flow is restricted between the motor's first end and the control chamber and there is a full flow path between the motor's first end and the bore to provide a force acting against the piston's first end. The control chamber connects with a sump. A drain valve entry port communicates with the motor's first end and an exit port thereof communicates with the sump to provide either entry port-exit port connection or blocking. A mechanism moves responsive to shifting of the control valve to shift the drain valve to connect the entry and exit ports when the control valve is in a Float or a Lower position and to block such connection in a Raise or Neutral position.

Abstract: A hydraulic actuator having two balanced area actuator pistons, each controlled by the movement of a primary servo sleeve to supply pressurized fluid from a pressure fluid source to one side of the actuator piston and to allow return flow from the opposite side of the piston. Piston movement is reversed by translation of the primary servo sleeve. A blocking valve is in each flow line between the servo sleeve and the cylinders in which each piston moves. A flow channel from the pressure fluid source applies pressure to the shut off valves to maintain the valves normally open for flow of control fluid to and from the actuator pistons. Means, operable upon loss of fluid pressure, are provided for closing the shut off valves to maintain existing pressure on both sides of the actuator pistons automatically to maintain reactive loads in each actuator half.

Abstract: A step-by-step controlled servomechanism of the type comprising a drive element movable provided with a plurality of receiver ports, and a fixed distributor to supply said receiver ports and having a number of transmitter ports capable of being connected, in pairs, respectively to the low pressure and to the high pressure, a fixed manifold having a plurality of manifold ports capable of communicating with the chamber of the drive element, all the receiver ports being capable of being made to open in succession into one of said manifold ports, selecting means capable of connecting, in succession by permutation and simultaneously, at least one pair of transmitter ports to the high pressure and to the low pressure, respectively, and at least one manifold port to the chamber of the drive element.

Abstract: A two-stage regulator valve, particularly for regulating the operation of single acting hydraulic lift motors in lift trucks, which embodies a spool valve controlling first and second ports in the valve body and responsive at one end to lift motor fluid pressure. At the opposite end the valve is responsive first to the force of a first spring which closes the spool on one port when lift motor fluid is below a predetermined pressure, and secondly, following predetermined compression of the first spring during which both ports are opened to exhaust pressure fluid from the lift motor, to the added combined force of a lift motor pressure responsive piston and a second spring. The piston forms with the spool valve a differential area response to said fluid pressure and together with the second spring regulates the position of the spool valve so as to variably restrict the other port to control the exhaust from the lift motor inversely as a function of the fluid pressure therein.

Abstract: A fluid cylinder assembly includes a cylinder and a piston slidably disposed in the cylinder, the cylinder having a cylinder bore closed at both ends by end caps. One of the end caps is provided therein with a flow control valve assembly for controlling the fluid flow from the cylinder bore during a working stroke of the fluid cylinder assembly. The flow control valve assembly comprises a poppet valve having a pair of poppets on a valve rod and a spool valve movable in response to the operation of the poppet valve. The poppet valve is shiftable by a solenoid alternately energized and de-energized by an electric control circuit which is closed just before contact of a cutter on the piston rod with a workpiece.

Abstract: The present invention relates to a control device for a controlled non-return valve provided in large hydraulic distributors. During the mode of operation where the double acting hydraulic ram is in a non-movable state, a means is utilized to communicate a control pressure to the one-way non-return check valve to positively maintain the check valve in a closed state thereby eliminating the accidental opening of the return valve and maintaining the double acting hydraulic ram in its initial position.

Abstract: A hydraulic system for opening and closing hopper barge comprises at least one and preferably two fluid pressure cylinders which are adapted to be positioned on one of the barge parts and a piston which is slidable in the cylinder which is adapted to be connected through its piston rod to the other barge part. The cylinder is connected by a first conduit at its piston rod end and by a second conduit at the opposite end, and a pump and a return line are connected to the respective first and second conduits. A pilot valve is located in the first and second conduits and includes three separate positions. In one position a pump discharge is connected to one end of the cylinder and the other end is connected to the return, and in another position, the opposite is true. In a third position, the first conduit is connected to a check valve and the pump discharge is connected to a return line in order to short circuit the pump.

Abstract: A cylinder locking apparatus primarily intended for use on counter-balanced lift trucks incorporating lift cylinders of the single-acting type, comprising a positive sealing poppet-type check valve which operates in the normal manner when the system is in the raise mode, but which in the lowering mode is operable by controlled pilot means triggered manually by the system's main control valve spool, thus permitting the load to be lowered by gravity.

Abstract: A baler adapted for towing by a tractor to pick up a windrow of hay and roll the hay into a cylindrical bale of substantial size and weight. The baler includes a gate pivotable rearwardly and upwardly to permit discharge of a completed bale onto the ground behind the baler. The gate is raised and lowered through a hydraulic system powered from the tractor. The hydraulic system includes a pair of hydraulic cylinders connected to the gate to raise and lower it upon extension and retraction respectively of the cylinders. This action is controlled by a manual flow control valve accessible to the operator on the tractor. The hydraulic system further includes a manual selector valve accessible on the baler to hydraulically lock the cylinders in their extended position to thus lock the gate in its raised position when desired.

Abstract: A main valve controls the flow to a load in a first or second direction depending upon the signal applied to a pilot valve. A spool of the main valve is biased to a central position blocking flow by springs. A throttle valve having an adjustable restrictor area is connected in one line between the main valve and the load. The restrictor area is adjusted in correspondence to an input signal. The spool is moved to a first or second extreme position by the pilot valve. The pressure difference developed across the throttle valve is applied to the spool in a direction to force the spool towards the central position. The spool is balanced at a predetermined pressure difference across the throttle valve.

Abstract: A hydraulic safety system for use with a hydraulic valve and a work apparatus which is retained under hydraulic pressure even when the hydraulic line is broken inadvertently. A lock valve is connected in the line and senses the loss of hydraulic pressure, and the valve is actuated in response to the pressure drop and retains the work apparatus in a pressurized condition to thereby avoid undesired movement in response to the loss of pressure.

Abstract: There is disclosed a hydraulic control system including body automatic and manual control for controlling the position of a grader blade for a motor grader. The hydraulic system is supplied fluid from a single pump and is provided with means to isolate the fluid supply from the automatic portion of the system when the manual portion of the system is activated.

Abstract: Step by step controlled servomechanism of the cylinder-type comprising a drive element movable in a case which it divides into two chambers and provided with a plurality of receiver ports, a distributor adapted to be put in communication with the high pressure and the low pressure and to supply said receiver ports, wherein said distributor is fixed and provided with a number of supply or transmitter ports at least equal to three but independent of the number of receiver ports, the transmitter ports being capable of being connected by permutation, in succession and in pairs, respectively to the low pressure and to the high pressure, the distance between the receiver ports and their length, on the one hand, the distance between the transmitter ports and their length, on the other hand, being such that, by the step by step displacement in one direction of the drive element, on one hand, it is possible to bring each time at least one receiver port between the two transmitter ports of a pair in such position that

Abstract: A load supporting hydraulic system having a pilot operated main control valve is provided with pilot controlled load check valve operative to permit free flow of fluid from said control valve toward the load supporting means of the system in response to pilot pressure and to block fluid flow from the load supporting means toward the control valve. The system includes vent valve means operative with the main control valve to permit the load check valve to be opened by fluid pressure in the load supporting means to allow fluid flow therefrom to the reservoir.

Abstract: A hydraulic load lifting system having a vented load check valve disposed in the main control line connected between the load supporting end of a hydraulic motor and a spool-type control valve to normally isolate the load generated pressures from the control valve is provided with venting apparatus which is operative to selectively prepressurize the main control line between the control valve and the load check valve prior to the venting of the load check valve and the shifting of the control valve to effect the lowering of a load being supported by the hydraulic motor so as to substantially balance the pressures on the opposite sides of the load check valve to prevent momentary load drop and hydraulic hammering.

Abstract: A fluid-powered control system of a type employing redundant control channels and multiple fluid signal amplifiers. Each fluid amplifier has two fluid outlets having communication with respective portions of a movable, force summing structure but having no communication with the other amplifiers. A respective, differential pressure monitoring device is connected between a source of fluid under pressure and each fluid amplifier and is operative to shut off fluid flow to the respective, associated, fluid amplifier upon the occurrence, across the outlets of the fluid amplifier, of a differential pressure exceeding a predetermined level.

Abstract: The invention is a control system for a hydraulic clamping device adapted to be connected to a source of pressurized hydraulic fluid and to a hydraulically operated clamp, which system comprises a path for supply of pressurized fluid to and from the clamp, a check valve in the said path, a hydraulically operated pilot piston movable between a non-actuating position and an actuating position in which it urges open the check valve, means to hold the pilot piston in the actuating position and thereby maintains the check valve open and a switching means for controlling the supply of pressurized fluid to and from the pilot piston, the switching means being capable of occupying a neutral position, a position in which pressurized fluid is supplied to the pilot piston to urge the pilot piston to the actuating position to open the check valve and a position in which pressurized fluid is supplied to the pilot piston to return the pilot piston to the non-actuating position and thereby permit the check valve to close.

Abstract: A lock valve assembly is provided in a hydraulic control system for a double acting hydraulic jack which is adapted to permit the desired substantially unrestricted flow of inlet fluid selectively to either end of the jack under all operating conditions and is automatically self-conditioning to either restrict when necessary or not to restrict the flow of outlet fluid from either end of the jack as may be required.

Abstract: An overrunning load control for a hydraulic jack includes a pair of variable orifice means in series, with the degree of metering of one of such orifices being determined by fluid pressure in the end of the hydraulic jack resisting the load. The degree of metering of the other variable orifice may be selectively chosen, through actuation of a relatively low pressure hydraulic pilot system.

Abstract: Control arrangement including an actuator, such as an air motor, for operating the jet deflector flaps of a thrust reverser, and a pilot valve and a control valve for operating the actuator. The pilot valve controls flow of high pressure air from the jet engine to the control valve. The air flows through a first port in the control valve to the actuator. Air leaving the actuator flows through a second port in the control valve and then back to the pilot valve through which it is exhausted. A pilot valve member initially limits air flow through the second port, but as the flaps are deployed, the second port is gradually opened and the first port gradually closed. The operation is reversed to retract the flaps. The pilot valve may be operated by a switch responsive to contact of the nosewheel of the aircraft with a runway. The pilot and control valve may be included in a single cylindrical body with their valve members rotatable about the axis of the body.

Abstract: A positioning apparatus includes a master hydraulic positioning cylinder having a relatively long stroke capability and a train of additively connected, short-stroke pneumatic control cylinders for controlling the length of stroke of the master cylinder and thus the positioning of a variable-position tool such as a band saw. One end of the cylinder train is connected by a feedback rod to an extensible portion of the master cylinder. A connecting rod connects the opposite end of the train directly to the spool of a three-position servo valve which controls the flow of hydraulic fluid to and from opposite sides of the master cylinder. When air pressure is admitted to a selected side of a selected control cylinder, at least a portion of the train shifts initially in a direction to move the servo valve to an operating position to begin stroking the master cylinder in a desired direction.

Abstract: A load supporting hydraulic system having a pilot operated main control valve is provided with pilot controlled load check valve operative to permit free flow of fluid from said control valve toward the load supporting means of the system in response to pilot pressure and to block fluid flow from the load supporting means toward the control valve. The system includes valve means operative independent of the main control valve to permit the load check valve to be opened by fluid pressure in the load supporting means to allow fluid flow therefrom to the reservoir. The system further includes safety valve means for blocking fluid flow to pilot control means.