Brake Failure, Warning Signals, and Emergency Procedures
Know the two warnings
Every air brake system warns you twice before the brakes quit. First, the low-pressure warning light and buzzer at about 60 psi - that is your signal to stop safely and find out why. Second, the spring brakes apply automatically somewhere between 20 and 45 psi: the yellow knob pops out and the rear wheels lock.
If the service brakes fail
- Keep steering and downshift to use engine compression.
- Look for an escape ramp and commit to it early.
- Use the parking (spring) brake gradually - pull the yellow knob slowly or make short applications, because it locks the rear wheels and can put you into a skid.
- If only one circuit has failed, the other still works. Brake gently and get off the road.
If brakes fade or pull
Fade is heat: stop, let the brakes cool, and check for dragging brakes before continuing. A pull to one side usually means a wet, misadjusted, or failed brake on that side.
Traps: the spring brakes are for parking and emergencies, not routine stopping - using them at highway speed can spin the truck. And if the low-pressure warning comes on while you are driving, do not "drive it to the shop": you may have only a few pedal applications of air left in the reservoir.
Rule of thumb: warning first, spring brakes second. Any time the buzzer sounds, treat the next safe pull-off as your destination.