Abstract: A tractor air brake system including front wheel brake actuators, rear wheel brake actuators, and control valve structure for controlling the service air pressure delivered to the front wheel brake actuators. When a tractor is equipped with this brake system, the control valve structure operates to vary the service air pressure delivered to the front wheel brake actuators relative to that delivered to the rear wheel brake actuators in order to minimize tractor front wheel lock and loss of steering control when operating a tractor-trailer combination and to provide safe stopping when operating the tractor only.

Abstract: A valve for a tractor air brake system for changing the pressure delivered to air brakes on the rear axle depending on whether the trailor is attached or whether the tractor is operating in the "bob tail" mode. According to the invention, a rear axle ratio control valve provides an automatic selection of input to output ratio in accordance with the presence or absence of trailor emergency pressure. In the ratio control valve, service brake control pressure always acts against a first effective area of a piston for providing a first ratio of input to output. A second effective area of the piston receives pressure with the presence of the trailer emergency so that the ratio of pressure delivered from the valve will be changed. Further in accordance with the invention, a blend-back valve operates at a higher, predetermined pressure range, even when the pressure communication is closed by the absence of the emergency pressure to enable the valve to deliver the maximum pressure at high input pressure.

Abstract: A tractor air brake system including front wheel brake actuators, rear wheel brake actuators, and control valve means for controlling the service air pressure delivered to the front wheel brake actuators. When a tractor is equipped with this brake system, the control valve means operates to vary the service air pressure delivered to the front wheel brake actuators relative to that delivered to the rear wheel brake actuators in order to minimize tractor front wheel lock and loss of steering control when operating a tractor-trailer combination and to provide safe stopping when operating the tractor only.

Abstract: A dual relay valve for controlling the supply of fluid pressure to two sets of brakes is operated by a single source of control fluid pressure. A solenoid-operated valve, controlled by a skid-control logic circuit isolates and vents the control fluid pressure upon detection of skid conditions.

Abstract: A master cylinder having a secondary piston for absorbing rapid initial pressure increases in proportion to the stroke of the primary piston. The master cylinder includes a housing having a first cylinder formed therein. A first piston is movable within the first cylinder. A second cylinder is formed in the first piston. A second piston is movable within the second cylinder.