Air Brakes and Brake System Checks on Combination Vehicles
Air Brakes and Brake System Checks on Combination Vehicles
The exam tests the combination-specific air brake checks: the dual air system, the tractor protection valve, trailer air supply, leak rates, and the order of the tests.
- Dual system. The tractor has primary and secondary circuits. The tractor protection valve keeps air in the tractor if the trailer breaks away or loses air.
- Trailer controls. Push in the red trailer air supply knob to charge the trailer. The yellow trailer hand valve (Johnson bar) applies trailer brakes only - use it to test, never to park or to make a normal stop.
- Build-up. With the engine at operating rpm, pressure should build from 85 to 100 psi within 45 seconds. The governor cuts out near 120-125 psi and cuts in near 85-100 psi.
- Static leak test. Engine off, brakes released: a combination may lose no more than 3 psi per minute (2 psi for a single vehicle). With the brakes fully applied, no more than 4 psi per minute (3 for a single).
- Low-air warning. The buzzer or light must come on before pressure drops below 60 psi.
- Spring brakes and protection valve. Parking spring brakes apply and the tractor protection valve closes between 20 and 45 psi.
- Trailer checks. Charge the trailer, shut off the engine, and watch the gauge for leaks; test the trailer brakes separately; check pushrod travel against the limit for the chamber (often 2 inches for a type 30 chamber).
Worked example. With the engine off and brakes released, the gauge falls from 100 to 92 psi in two minutes: 8 psi divided by 2 minutes equals 4 psi/min, above the 3 psi/min combination limit. The vehicle fails the test - find and fix the leak before driving.
Traps that cost marks. Running the engine during the static test; using the 2 psi single-vehicle limit for a combination; pumping the pedal during an ABS stop; assuming a trailer ABS light means the trailer brakes are adjusted.