When the brakes are engaged, air from the brake valve enters into the quick-release valve, forcing the diaphragm down and closing off the exhaust port. This action allows air pressure to rush through the quick-release valve outlet ports to the wheel brake chambers. When the brakes are released, the air pressure above the quick-release diaphragm is

Figure 7-48.- Trailer control valve.

Figure 7-49.- Quick-release valve.

exhausted at the brake valve. As air pressure above the diaphragm is released, the air pressure below the diaphragm raises off the exhaust port. This action allows the air in the brake chambers to exhaust at the quick-release valve.

When air is leaking from the system, a leakage test can determine if there is air leaking at the quick-release valve. The leakage test is performed with the brakes applied and coating the exhaust port with soapsuds. If air bubbles form, this is a sign of a defective valve, which can be corrected by either cleaning and replacing worn parts or by replacing the unit. Dirt, worn diaphragm, or a worn seat causes leakage.

Combined-Limiting and Quick-Release Valve
The combined-limiting and quick-release valve (fig. 7-50) is used in combination with a two-way check valve in the air brake system of trucks and tractors. The combined-limiting and quick-release valve is interchangeable in mounting with the quick-release valve and serves the same purpose with the additional function of providing an automatic reduction of front-wheel brake pressure, at the option of the operator, on slippery roads.