An arrangement for use with a valve system permits facilitated adaptation to conditions where modified lever ratios are desired while an actuation force requirement remains unaffected, or where a modified actuating force requirement is desired while lever ratios remain unchanged. In order to avoid or reduce costs associated with modification of the actuation force requirement of the valve system, a spring is provided which acts upon an operating element thereof, in addition to a pressure force, and codirectional therewith, against the actuating force in such manner that a predetermined actuating force requirement results. The invention finds particular significance in applications directed to vehicle braking systems.

The invention relates to a level control arrangement for vehicles having air springs (6a) to (6d) and a pneumatically controllable directional valve (26). A residual pressure holding function and an overpressure function are integrated into the directional valve (26). The directional valve (26) is controlled by the air pressure in the air springs (6a) to (6d). The air pressure can be applied via a control line (20) to a control input (24) of the directional valve (26). The air from the air springs (6a) to (6d) is released with the aid of the venting line (28). The venting line (28) is guided separately from the control line (20) through the directional valve (26). In this way, a large air flow can be conducted through the venting line (28) without the static air pressure in the control space (50) of the directional valve (26) being reduced. The venting line is blocked by a stepped piston (44) of the directional valve when no air is to be released from the air springs.

An air brake system for a vehicle having front service and parking brakes, and rear service and parking brakes, includes separate front service, rear service and parking air systems. In the event of a failure in either service air system, normal operation of the service brake control will automatically operate the parking brakes associated with the failed service brakes.

A vehicle brake system includes a source of fluid pressure adapted to supply a braking force a brake actuator including a parking brake chamber, and a combination valve fluidly coupling the brake actuator to the source of pressure, the combination valve having an inlet fluidly coupled to the source of pressure and an outlet fluidly coupled to the brake actuator. The combination valve includes within a single housing (i) a solenoid responsive to electrical signals to move from one position in which the parking brake chamber is fluidly coupled to the source of pressure in order to pressurize the parking brake chamber and release the parking brake, and another position in which the parking brake chamber of the brake actuator is vented in order to exhaust pressure within the parking brake chamber so that the parking brake is applied, and (ii) at least one of a manifold, an inversion valve, a double check valve and an anti-compounding valve.

A fluid pressure braking system for a tractor-trailer combination vehicle includes a proportioning relay valve which controls communication to the rear wheel brakes of the tractor. When the tractor pulls a trailer, the proportioning control within the relay valve is disabled, but when the vehicle is operated in the "bobtail" mode without pulling a trailer a proportioned brake application is effected in which the braking pressure communicated to the rear brake actuator is a predetermined percentage of that in the front wheel braking circuit. The proportioning mechanism is also disabled when the front axle braking circuit malfunctions, so that upon loss of pressure of the front axle system, the driver effects a non-proportioned rear brake application.