The most common cause of loss of braking is poor brake adjustment. The popular type-30 air chamber has 2 1/2 in. (63.5 mm) of available stroke. A correctly-adjusted brake will have 1/2 in. (12.7 mm) to 3/4 in. (19 mm) of slack, leaving two in. (50.8 mm) of reserve chamber stroke. When slack reaches 3/4 in. (19 mm) the brakes MUST be adjusted. This is the most important 3/4 in. (19 mm) of your life.
- At an 80 psi (552 kPa) application, a brake chamber with 3/4 in. (19 mm) of slack will stroke 1 3/4 in. (44.5 mm) due to component stretch. This reduces reserve chamber stroke to 3/4 in. (19 mm).
- Cast iron expands when heated. On a hot brake drum this can cause the chamber to stroke a further 1/2 in. (12.7 mm), reducing reserve stroke to 1/4 in. (6.4 mm)
- At high temperature, brake lining wears rapidly. Lining wear the thickness of three sheets of paper causes the chamber to stroke a further 1/4 in. (6.4 mm), resulting in the chamber “bottoming out” and a probable runaway.
- Even with cold drums, a vehicle with poorly adjusted brakes will have up to a 75% longer stopping distance than normal (Table 1).
CAUTION: Under normal light braking conditions even grossly maladjusted brakes seem to respond satisfactorily. It is only under moderate to heavy braking that this dangerous condition will become apparent.
Vehicle: 6x4 truck | Weight: 55, 000 lb. | Speed: 60 mph
|Average stopping distance (feet)|
|Brake lining temperature||Fully-adjusted brakes||Back off to limit||Increase|
|150 F||342 ft||458 ft||34%|
|200 F||351 ft||519 ft||48%|
|300 F||366 ft||625 ft||71%|
|400 F||393 ft||692 ft||76%|