Air Brakes on Combination Vehicles
What this domain tests
The exam tests your knowledge of the tractor-trailer air system: compressor and governor operation, dual reservoirs, low-pressure warning, spring brakes, leakage tests, and daily inspection of slack adjusters and hoses.
Core rules
- The compressor builds pressure; the governor cuts it out and back in. A dual system keeps steering and rear service brakes separate enough to retain some braking after a failure.
- A low-pressure warning must activate before pressure falls below 60 psi. Spring brakes apply automatically at roughly 20-45 psi.
- With engine off and brakes released, combination leakage must not exceed 4 psi in one minute. With brakes fully applied, it must not exceed 6 psi in one minute.
- Drain air tanks daily and check that push rods and slack adjusters move properly.
Worked example
After building full pressure, you shut the engine off and release the parking brake. The gauge drops 5 psi in one minute. On a combination vehicle the released-brake limit is 4 psi, so this fails. You then fully apply the foot valve and see a 7 psi drop; the applied-brake limit is 6 psi, so that also fails. The reasoning: combination vehicles carry more air lines and chambers, so the allowed leakage is slightly higher than for a single vehicle, but these readings exceed even the combination limits.
Traps that cost marks
- Applying single-vehicle leak limits to a tractor-trailer.
- Testing only the tractor and ignoring trailer air lines and chambers.
- Ignoring a low-pressure warning or assuming spring brakes are only for parking.
- Skipping the applied-brake leak test.