Inspecting Air Brake Systems
The inspection sequence
This domain tests the standard air brake tests and the numeric limits. Inspect on level ground, with the system fully charged, engine off, and parking brake released unless the test says otherwise.
- Compressor build-up: at fast idle, pressure should rise from 85 to 100 psi within about 45 seconds on a typical CDL test vehicle.
- Static leakage: with engine off, parking brake released, and system charged, watch for one minute. Allowable loss is about 2 psi for a single vehicle and 3 psi for a combination.
- Applied leakage: hold the foot valve fully applied. Allowable loss is about 3 psi per minute single and 4 psi per minute combination.
- Low-pressure warning: the buzzer and light must activate by 60 psi. The spring brakes should apply between about 20 and 45 psi.
- Pushrod stroke: compare each chamber’s stroke with its stamped adjustment limit. Stroke limits vary by chamber type and size, so a single number for every chamber is not reliable.
Worked example
A combination vehicle is charged, engine off, parking brake released. In two minutes the gauge falls from 100 to 93 psi. That is 3.5 psi per minute. The combination limit is 3 psi per minute, so this fails. With the pedal fully applied, a drop of 9 psi in two minutes is 4.5 psi per minute, also over the 4 psi combination limit. Both results require repair before dispatch.
Traps
Leaving the engine running invalidates a static leak test. Testing with the parking brake set can hide leaks. Using single-vehicle limits on a combination vehicle is a common scoring error. Do not skip the warning-light check or the tank drains.