A 20-Minute Pre-Trip With Invalid Air Tests
Before leaving the terminal, a driver has 20 minutes to do the air brake pre-trip checks on a single-unit truck. The tank is at 100 psi when he starts. He first starts the engine, presses the brake pedal, and watches for 1 minute; the gauge drops from 100 to 94. He then fans the brakes until the low-air warning sounds at 52 psi and the spring brakes apply at 30 psi. Next he builds pressure back to 100 psi, shuts off the engine, releases the parking brake, and checks for a released-brake leak; after 2 minutes the gauge is at 96 psi. He does not check compressor build-up time and says the vehicle passes because the warning and spring brakes work.
1. Identify the errors in this sequence and explain how each one makes a test result unreliable.
Read the worked answer
Several errors make the results unreliable. The applied leak test was done with the engine running, so the compressor can replenish air and hide leakage; the engine must be off. The test started at 100 psi rather than full governed pressure, so there is less air and the system may not be at a stable, fully charged condition. Fanning the brakes before the leak tests drained the tanks and changed the starting point for later checks. The released leak test was also started at 100 psi, not at cut-out, and the compressor build-up time was never checked. The low-air warning at 52 psi and spring brakes at 30 psi are within the expected ranges, but those results do not validate the leak tests.
2. Describe the correct order for the air brake pre-trip tests and the condition each test requires.
Read the worked answer
With the engine running, build to governor cut-out and check compressor build-up against the shop specification. Shut off the engine, chock the wheels, release the parking brake, let the gauge stabilize, and perform the released-brake leak test. Then press and hold the pedal firmly and perform the applied-brake leak test. After that, fan the brakes down to confirm the low-air warning activates before 60 psi and the spring brakes apply in the 20-45 psi range. Finally, restart the engine and rebuild pressure. Each leak test needs the engine off and a fully charged system; the warning and spring-brake check comes after the leak tests because it deliberately uses up air.
3. If the corrected tests show the warning at 52 psi and spring brakes at 30 psi, do those results pass? Does the vehicle pass overall?
Read the worked answer
Those two results pass: the warning must come on before 60 psi, and 52 psi meets that; spring brakes should apply in roughly the 20-45 psi range, and 30 psi is inside it. But the vehicle does not pass overall unless the released and applied leak tests and the compressor build-up test also meet their limits. A correct warning and spring-brake application only show that part of the system works; they say nothing about excessive leakage or a weak compressor.