Air Brakes
Air Brakes tests the system that stops a heavy passenger vehicle when hydraulic brakes would not be practical. You need to know the components, the meaning of pressure readings, and the sequence of the CDL air-brake tests.
System basics
- A dual air brake system has two separate circuits; a failure in one should leave some braking capability.
- The compressor charges the reservoirs. A governor controls cut-in and cut-out. The safety valve opens at about 150 psi to protect the system.
- A low-pressure warning must activate by 60 psi. Spring brakes apply between about 20 and 45 psi when pressure falls too low.
Tests and limits
- Build pressure from 85 to 100 psi within 45 seconds.
- With the engine off and parking brake released, leakage should be less than 2 psi per minute for a single vehicle (less than 3 psi for a combination).
- With the service brake applied, leakage should be less than 3 psi per minute for a single vehicle (less than 4 psi for a combination).
- Know the pushrod travel limits and check slack adjusters; do not adjust a slack adjuster that is not free.
Worked example
After building to 120 psi, you shut off the engine, release the parking brake, and watch for one minute. The gauge drops from 120 to 118 psi. That is a 2 psi loss, acceptable for a single vehicle. A 4 psi loss would mean a leak: identify it and do not drive until repaired.
Traps
Running the leakage test with the parking brake applied; confusing service-brake and parking-brake checks; ignoring a compressor that takes too long to build pressure; and fanning the brakes on a long downgrade.