Inspecting and Using Parking Brakes and Spring Brakes
This domain tests the parking brake control, the spring brake mechanism, and the rules for parking safely. Expect questions on the yellow knob's operation, on caging bolts, and on when not to set a parking brake.
Controls and operation
- The yellow diamond-shaped knob is the parking brake control: pull out to apply, push in to release once pressure is up.
- Spring brakes are held off by air pressure; when air is released, heavy springs apply the brakes mechanically.
- A modulating (dual) valve lets the driver apply the spring brakes gradually for an emergency stop instead of a full lockup.
- A caging bolt mechanically compresses the spring so a brake can be released for towing or repair when there is no air. It must be removed and the brake restored before the vehicle returns to service.
Parking rules
- Always set the parking brake and chock the wheels when parked, especially on a grade.
- Do not set a parking brake on very hot drums - trapped heat can crack drums and damage linings. Let them cool first when possible.
- Never use the parking brake as a normal service brake; it is for parking and emergencies only.
- Before releasing, confirm full air pressure; otherwise the vehicle will not move or will drag.
Inspection points
- Check that the knob applies and releases the brakes and that the valve does not leak.
- Check the push rod stroke; excessive travel on a manual slack adjuster means adjustment, and stroke beyond the chamber limit is an out-of-service condition.
- Confirm the parking brake holds the vehicle against a light throttle on a grade.
Worked example
A truck will not move after sitting overnight in cold weather, and the yellow knob is out. Reasoning: the spring brakes are applied. The fix is to build full air pressure and push the knob in - not to cage the brakes, which removes the parking brake entirely.
Traps that cost marks
Pushing the knob to apply; using a caging bolt as a permanent fix; setting a parking brake on glowing drums; assuming a slack adjuster that moves freely is correctly adjusted.