Air Brakes and Brake Systems for Combinations
How the System Works
A combination vehicle uses a tractor protection valve, relay valves, and a dual air system. The compressor builds pressure, and the governor cuts it off at a set maximum. The tractor protection valve closes the tractor's air supply to the trailer if pressure drops too low or if the trailer breaks away. The trailer has spring brakes that apply when air pressure is lost, and service brakes that apply when you press the pedal.
Pre-Trip Air Checks
You must know the standard tests: the static leak test (engine off, pressure should not drop more than a set amount in one minute with brakes released, and a bit more with brakes applied), the applied leak test, the low-air warning test (warning buzzer or light should activate before pressure falls to about 60 psi), and the spring brake test (with pressure below about 45 psi, the spring brakes should apply). Traps: confusing the warning threshold with the spring-brake pop-out pressure, and forgetting that the tractor protection valve is what isolates the trailer.
In-Service Behavior
If the low-air warning sounds while driving, stop safely and do not continue until the problem is corrected. If the trailer air line separates, the emergency line loss triggers the trailer spring brakes. Know that the service line loss alone does not automatically set the trailer brakes.
Rule of Thumb
"Warning at 60, springs at 45." Memorize those two numbers and what each one means.