This section covers the operation and testing of the air brake valves. These valves include the single check valves, the double check valves, the dual application valves, the moisture ejector valves and the pressure protection valves. The air valves are pneumatic controls that route compressed air through the hoses and the lines to or from the brake components.

All the air valves have air passages and internal moving parts. As the internal parts move, they direct the compressed air to the desired location. Each valve has at least one supply port to allow air to enter the valve and one delivery port to allow air to exit the valve. Some valves have a control port that allows an air pressure source to control when the air can flow through the valve. An exhaust port allows air to vent to the atmosphere.

The air brake valves operate either manually or automatically. The manual valves require the driver to move a knob, a handle or a pedal in order to work the valve. The automatic valves work by using the control ports or internal regulating parts.

The air brake system is used only on the P92 chassis. The foundation brakes are hydraulic. The actuation and booster function are accomplished with air pressure. The application of the brakes is controlled by a dual circuit application valve. The application valve receives air from a dual air reservoir. The main reservoir is supplied by a single cylinder air compressor. The front and the rear service brakes have a separate air system with the air flow controlled by different portions of the dual application valve. The gauges located in the dash indicate the air pressure in the front and the rear service air systems.

When the brake pedal is depressed, the plunger is pushed in on the application valve. Air pressure exits from the two circuits of the valve to the brake intensifiers. The pistons in the intensifiers are pushed inward, supplying hydraulic pressure to the calipers. Pistons in the calipers are forced outward pushing the brake linings against the brake rotor. The action slows and/or stops rotation of the wheel assembly.