from the brakes and work together to ensure control and safe braking application. These valves are as follow:

Treadle valve (brake valve)

Trailer control valve (brake valve)

Quick-release valve

Combined-limiting and quick-release valve

Tractor protection valve

Relay emergency valve

Check valves

In the following paragraphs we will discuss each valve in more detail.

Treadle Valve

The treadle valve (fig. 7-47) controls the air pressure delivered to the brake chambers. When the treadle valve is depressed, force is transmitted to the pressure regulating spring and diaphragm that is moved downward and contacts the exhaust valve and closes it. Continued movement opens the inlet valve and air pressure from the reservoir flows through the valve and into the delivery lines to apply the brakes. As the air pressure increases below the diaphragm, it overcomes the force above the diaphragm and the diaphragm raises slightly. This action allows the inlet valve to close but also keeps the exhaust valve closed, thereby obtaining a balanced position. Further depression of the treadle valve increases the forces above the diaphragm and correspondingly increases the delivered air pressure until a new balanced position is reached.

Maintenance of the treadle valve consists of periodic lubrication of the hinge and roller. Should the valve malfunction, it can be disassembled and cleaned. After cleaning, the internal parts should be lubricated with Vaseline before reassembly. This prevents moisture in the air system from causing corrosion and freezing of the valve. If cleaning does not remedy the malfunction, the valve must be replaced.

Figure 7-47. - Treadle valve.