Colorado CDL Air Brakes: Air Brake Inspection and Leak Checks

Domain 2, Air Brake Inspection and Leak Checks, tests the order and pass/fail limits of the pre-trip air brake test. In Colorado, this is the sequence an examiner watches you perform, so know what to do, when to do it, and the numbers that make a vehicle unsafe.

Test sequence

1. Chock wheels, drain tanks, check compressor build-up from 85 to 100 psi at fast idle (about 45 seconds).
2. Let pressure reach governor cut-out, shut off engine, release parking brake (chock wheels first).
3. Static leak test: watch both gauges for one minute. Loss should be no more than 3 psi for a single vehicle and 4 psi for a combination.
4. Applied leak test: fully apply foot valve, hold one minute. Loss should be no more than 4 psi single, 6 psi combination.
5. Pump down with engine off; low-pressure warning must sound before 60 psi.
6. Keep pumping; spring brakes should set at 20-45 psi.
7. Tug test: with parking brake applied, gently pull against it in low gear to confirm it holds.

Also inspect slack adjusters, push rods, hoses, chambers, drums, linings, and the ABS lamp.

Worked example

A combination vehicle loses 5 psi on the static test and 7 psi on the applied test in one minute. Both exceed the limits (4 psi static, 6 psi applied), so the vehicle fails. Reasoning: a leak that shows up with brakes applied but worsens when applied suggests a service-side leak, such as a chamber diaphragm, hose, or fitting. The driver should not drive; find the leak with soapy water and repair it.

Traps

Testing with the engine running hides leaks because the compressor keeps rebuilding pressure. Forgetting to release the parking brake gives a false reading. Watching for only 30 seconds is not the test. Do not confuse the static and applied limits. And never crawl under a vehicle that is not chocked.