Abstract: A housing of the apparatus has a device for connection to a source of pressurized air, and is provided with first and second connectors for connection to brake system lines. A compressed air tank is located in the housing, and at least three double manometers are provided. Fluid couplings are connected with the manometers. A first fluid line connects the air tank with the device and has a one-way valve and an adjustable pressure regulator interposed in it, a second fluid line connects the air tank with the first connector, and a third fluid line connects the air tank with the second connector and has a one-way valve interposed in it. A multi-position manually operable air spring regulating valve is interposed in the second line and has a valve body, an operating handle for moving the valve body between two operative end positions and a centering-spring arrangement for centering the handle in a rest position in which the valve body is in an intermediate blocking position.