Air Brakes and Braking Systems
Air Brakes and Braking Systems
This domain tests the air-brake system on a combination vehicle: compressor and governor, reservoirs, foot valve, relay valve, spring brakes, tractor protection valve, and the red and blue trailer lines. Expect questions on pre-trip air-brake tests and on what happens when pressure falls.
Core rules
- The compressor governor controls when the compressor cuts in and out. Build pressure from 50 to 90 psi within 3 minutes at fast idle.
- The low-pressure warning must activate by 60 psi. Spring brakes begin to apply, and the tractor protection valve closes, at about 20 to 45 psi.
- The red glad hand is the emergency/trailer air-supply line; the blue glad hand is the service line.
- With a fully charged system and the engine off, release the service brake and watch for no more than 3 psi loss in one minute. Then apply and hold the brake; loss must not exceed 4 psi in one minute.
Worked example
A driver charges the system, shuts off the engine, and releases the parking brake. After one minute the gauge has dropped 2 psi, which passes the 3-psi limit. The driver then holds the service brake for one minute and sees a 6-psi drop. This fails because the applied-brake limit is 4 psi; the vehicle needs repair before dispatch.
Traps that cost marks
Do not fan the brakes or ignore a low-pressure warning. Do not confuse the red and blue lines, and do not test leaks with the engine running. Remember that the tractor protection valve is designed to keep air from escaping from the tractor if the trailer separates or a line fails.
Key points
- Build 50 to 90 psi within 3 minutes at fast idle.
- Low-pressure warning must work by 60 psi.
- Spring brakes and tractor protection activate around 20 to 45 psi.
- Red is emergency/trailer air supply; blue is service.
- Leak limits: 3 psi released, 4 psi applied, in one minute.