When the pedal is released, air is forced from the brake chamber by the brake shoe return spring acting on the linkage. After the shoes reach the fully released position, the return springs acting on the diaphragm causes it to return to its original position in the chamber.

When performing maintenance of the brake system, check the brake chamber alignment to avoid binding action. Check the pushrod travel periodically, and when necessary'. adjust the brakes so that pushrod travel is as short as possible without the brakes dragging. The pushrod length should be adjusted so that the angle between the center line of the slack adjuster and the brake chamber pushrod is 90 degrees when the pushrod is extended to the center of its working stroke.

SLACK ADJUSTERS
The slack adjusters (fig. 7-46) function as adjustable levers and provide a means of adjusting the brakes to compensate for wear of linings. Air pressure, admitted to the brake chamber when the brake pedal is depressed, moves the slack adjuster toward the position indicated by the dotted lines.

The entire slack adjuster rotates as a lever with the brake camshaft, as the brakes are applied or released. Turning the adjusting screw makes the brake adjustments necessary to maintain proper slack adjuster arm travel (shoe and drum clearance). This action rotates the worm gear, camshaft, and cam. expanding the brake shoes so that the slack caused by brake lining wear is eliminated and the slack adjuster arm travel is returned to the correct setting. The movement of the cam forces the brake shoes against the brake drum. Friction of the brake lining against the drum stops the turning movement of the wheel. When the brakes are released, the brake shoe return spring pulls the shoes back to a DISENGAGED position.

BRAKE VALVES
There are numerous brake valves used in an air brake system. These valves either apply or release air

Figure 7-45.- Brake chamber.

Figure 7-46.- Slack adjuster.

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.

Trailer Control Valve
The independent trailer control valve (fig. 7-48) provides the operator with control of the trailing load at all times. This valve functions in the same manner as the treadle valve except that the handle is turned, rather than depressed, to operate the valve.