Air Brakes and Combination Vehicle Brake Systems
What the exam tests
Air brakes on a combination vehicle are two braking systems joined by control valves. The exam asks about the dual air system, the tractor protection valve, trailer air lines, leak tests, low-pressure warning, and spring brakes.
Rules to know
- The compressor charges the reservoirs; the governor cuts out near 120–125 psi and cuts in around 100 psi.
- Blue glad hand = service line. Red = emergency line. Crossed lines can release trailer brakes or prevent charging.
- The tractor protection valve closes the emergency line if tractor pressure falls to about 20–45 psi, applying the trailer spring brakes.
- Static leak test: engine off, parking brakes released, system fully charged. Loss should not exceed 4 psi per minute on a combination; the single-vehicle limit is 3 psi.
- Applied leak test: hold the brake pedal; loss should stay within the manual’s limit, commonly 4 psi per minute.
- Low-pressure warning must sound before 60 psi. Spring brakes should engage in the 20–45 psi range.
- Check slack adjusters and push rod travel. A long stroke means weak braking.
Worked example
After building to cut-out, you shut the engine off and release the parking brake. In one minute the gauge falls from 120 to 113 psi. That is 7 psi, above the 4 psi combination limit. Reasoning: a leak exists in the tractor or trailer circuit. Do not drive; find and fix the leak, then repeat the test.
Traps that cost marks
- Using the single-vehicle 3 psi limit for a combination.
- Testing with the parking brake applied.
- Assuming the trailer is charged because the lines are connected.
- Ignoring a slow leak that appears only when the pedal is held.