The most common cause of brake failure 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 has only 1/2 in. (12.7 mm) of slack, leaving 2 in. (50.8 mm) of reserve chamber stroke. When slack reaches 1 in. (25.4 mm) the brakes must be adjusted. This is the most important inch of your life.
Here’s why:
- At an 80 psi application, a brake chamber with 1 in. of slack (25.4 mm) strokes 1 3/4 in. (44.5 mm) because of component stretch. This reduces reserve chamber stroke to 3/4 in. (19.1 mm).
- Cast iron expands when heated. On a hot brake drum, this can cause the push rod to stroke a further 1/2 in. (12.7 mm), reducing reserve stroke to 1/4 in. (6.4 mm).
- Brake lining wears rapidly at high temperatures. If the lining wears down just the thickness of three sheets of paper, the push rod strokes a further 1/4 in. (6.4 mm). This could cause the chamber to bottom out and your brakes to fail.
- 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% |