Air Brake Manual

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Within an inch (25 mm) of your life

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.

Here’s why:

  • 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 temperatureFully-adjusted brakesBack off to limitIncrease
150 F342 ft458 ft34%
200 F351 ft519 ft48%
300 F366 ft625 ft71%
400 F393 ft692 ft76%


Rev: 2018