Leak-Down Test Fails After Ten Minutes
A driver performs an air brake leak check on a 2018 tractor-trailer after charging the system to governor cut-out at 120 psi. He shuts off the engine, releases the parking brake, and applies and holds the service brake pedal for one minute. The gauge drops from 120 to 111 psi. He repeats the test with the service brake released and gets a drop from 120 to 114 psi in one minute. The tractor is a combination vehicle. The driver hears a faint leak near the front steer axle but cannot see it. The outside temperature is 35°F and the air dryer purge valve cycles normally. The driver has a 300-mile run and a load of produce. The low-pressure warning does not come on during the test. The trailer supply line is connected and the trailer brakes are released for the test.
1. Apply the correct leak-rate standard for this combination vehicle and interpret both readings.
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For a combination vehicle, the usual CDL standard is:
- Brakes released (static) test: loss should not exceed 3 psi in one minute.
- Service brake applied and held: loss should not exceed 4 psi in one minute.
Here the released-brake loss is 120 − 114 = 6 psi/min, and the applied-brake loss is 120 − 111 = 9 psi/min. Both exceed the limits, so the vehicle fails the leak check. The fact that the low-pressure warning did not sound is not reassuring; the warning is required to activate before pressure falls below 60 psi, and this test started at 120 psi and lasted only one minute. The system has a real leak, likely near the front steer axle, and it should not be dispatched until repaired.
2. What is the correct test procedure, and what errors could make a leak test misleading?
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Correct procedure:
- Charge the system to governor cut-out, about 100–125 psi, then shut off the engine.
- Release the parking brake and confirm the wheels are chocked.
- Let the gauge stabilize, then observe for one minute with the service brake released. Record the drop.
- Apply and hold the service brake fully for one minute. Record the drop.
- Compare with the combination-vehicle limits: 3 psi released, 4 psi applied.
Errors that can mislead:
- leaving the engine running or the compressor cycling;
- not releasing the parking brake for the released test;
- not allowing pressure to stabilize;
- applying the brake only lightly instead of fully;
- testing with the trailer supply line disconnected or trailer brakes applied;
- reading the wrong gauge or testing a cold system with ice-blocked lines.
A soapy-water check should follow at the front steer chamber, hose, fittings, foot valve, relay valve, and gladhands. Do not use hands to feel for leaks.
3. Why is a front steer axle leak especially important, and what should the driver do before the 300-mile run?
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A front steer axle leak can affect a front service chamber or line. If it is a service diaphragm leak, the front brake may be weak or may drag; if it is a supply leak, pressure can drop faster when the brakes are applied. Either way, steering-axle brake imbalance can cause a pull or skid during hard braking, and a sudden loss can trigger the low-pressure warning and spring brakes on the rear. The driver should not start the 300-mile produce run. He should chock the wheels, build pressure, and use soapy water to pinpoint the leak; then repair or replace the chamber, hose, or fitting and repeat both leak tests. Only after the system holds within limits and the low-pressure warning and spring brakes are verified should the truck leave.