A trailer air braking system has combined spring brake and service brake actuators, and the service brake chambers of the actuators are controlled by a combined emergency relay and modulator valve assembly of known construction. In order to ensure that the spring brake function does not become operative due to a moderate loss in the pressure of the trailer reservoir, such as would occur in the event of an anti-skid test in which the reservoir is isolated from the emergency line and the modulator valve is cycled, a spring brake one-way control valve is inserted into the spring brake circuit leading from the reservoir. The control valve acts in one condition as a one-way valve to prevent air returning from the spring brake actuator chamber to the reservoir, but on a large loss in reservoir pressure, a piston which carries the one-way valve seat and is subjected to reservoir pressure moves under spring biassing such that the one-way poppet valve member is unseated by a control pin to permit a reduction in spring brake pressure.

A dual pressure vehicle air brake system (60, 61, 100, 101) made according to the invention includes first and second sources (62, 64) of pressurized air at, for example, 175 psi and 100 psi, respectively. A combination brake actuator (6) includes a conventional service brake chamber (8) adapted to accommodate the lower, second air pressure while the spring brake chamber (10) is made to accommodate the higher, first pressure. The use of the higher first pressure allows a stiffer actuator spring to be used and be fully deflected by the first pressure without substantially changing actuator piston areas and other elements of conventional combination brake actuators. A conventional modulated spring brake control valve (78) is used to supply air at the first pressure to the spring brake chamber. The service brake chamber is supplied with air at the second pressure through the outlet of a conventional service brake application valve (68). Compounding of brake forces, which could occur if both the service and spring brakes were applied simultaneously, may be reduced or eliminated using a conventional two-way check valve (50 ) or a novel shut off valve (102).

An air brake control system for use in truck trailers having at each axle compressed-air brakes including spring brakes and service brakes, and each trailer being coupled to receive compressed air through an emergency (supply) gladhand and to receive service brake signals through a service (control) gladhand, the invention comprising an integrated brake valve system for each axle of each trailer, including a common valve housing; a spring brake valve cluster having a non-return check valve connected to receive and pass air pressure from the emergency gladhand and to deliver it directly to the spring brakes and an air pressure responsive shuttle operative to open the check valve whenever the emergency line pressure falls below a predetermined spring brake isolation pressure above which the spring brake holding pressure is not affected by fluctuations of the emergency gladhand pressure; and including an air tank supply valve cluster connected between the emergency gladhand and the tank and operative to sense pressure in the emergency gladhand and to open at a tank-filling threshold pressure above the predetermined spring brake isolation pressure to pass air to the tank and having a tank protecting check valve to prevent backflow of air from the tank; and including a pressure amplifying service brake relay valve cluster connected between the tank and the service brakes to supply and modulate air to the latter responsive to variations in the service gladhand pressure.

The instant invention is a positive shutoff valve for use on emergency vehicles so as to maintain air within a reservoir tank allowing immediate movement of an emergency vehicle upon start up. The shutoff valve is operated by a switch located in the operator's compartment of the emergency vehicle providing movement of a solenoid located in the shutoff valve which is biased in a normally closed position. A manual bypass lever is provided should the electric solenoid fail to operate correctly. The shutoff valve positively maintains air within the reservoir tank and is monitored by a low pressure switch and alarm sensor which will indicate low air pressure in the reservoir tank even when the vehicle is in an off position.

A double check valve is integrally located in a spring brake chamber to reduce plumbing associated with a pneumatic brake system. Anti-compounding is provided by virtue of a bias pressure of approximately twenty to thirty psi between a pressure chamber of a service actuator and an operating chamber of a parking/emergency actuator. Likewise, vehicle roll-back is addressed since the transition from a service application to a park application assures that the service brakes are sufficiently applied until the spring brake becomes operative.