Brake Adjustment, Warning Devices, and Malfunctions
Domain 3, Brake Adjustment, Warning Devices, and Malfunctions, tests whether you can measure push rod travel, recognize warning devices, and respond correctly when pressure or braking fails. Colorado exam questions often turn on the difference between an out-of-adjustment brake and a failed component.
Brake adjustment
Measure push rod travel with the brakes fully applied and system pressure at 90-100 psi. Compare travel to the limit for the chamber type; a common limit for a Type 30 clamp-type chamber is 2 inches. If travel exceeds the limit, the brake is out of adjustment. Manual slack adjusters can be adjusted; automatic slack adjusters should not need routine manual adjustment-if one does, it is a sign of a worn or failing component, so repair it rather than crank on it. With brakes released, pull each slack adjuster by hand; movement of more than about 1 inch at the push rod indicates a problem.
Warning devices and malfunctions
- Low-pressure warning: light and buzzer before 60 psi. If it comes on while driving, stop as soon as it is safe; spring brakes will apply as pressure falls.
- ABS lamp: if it stays on after start, ABS is disabled but normal service brakes remain. Drive to repair; do not pump the brakes.
- Parking brake control: yellow diamond knob; pull to apply, push to release.
- Governor failure: pressure climbs past cut-out; safety valve releases near 150 psi. Stop and repair.
- Compressor failure: pressure falls; warning sounds; spring brakes apply. Get off the road.
Worked example
At 95 psi with the brake pedal fully applied, a Type 30 clamp chamber's push rod extends 2 3/8 inches. The limit is 2 inches, so it is overstroked and out of adjustment. Reasoning: overstroke reduces brake force and can overheat the chamber; the vehicle must be adjusted or repaired before dispatch, not driven on a short trip.
Traps
Measuring travel without applying the brakes or at low pressure gives a false pass. Do not manually adjust an automatic slack adjuster to hide a wear problem. Do not ignore an ABS lamp just because the truck still stops. And never treat the yellow parking knob as a normal service brake.