Air Brake System Maintenance and Adjustments
Adjustment questions are about measurement, not feel, and the exam expects you to know when to turn the nut and when to find the underlying fault instead.
Adjustment basics
Brake adjustment is measured as push rod travel. With the system at 90–100 psi and brakes released, mark the push rod, apply the brakes fully, and measure travel. Each chamber has a limit stamped on it (commonly 2 inches standard stroke, 2½ inches long stroke for type 30 chambers). Beyond the limit the slack adjuster has used up its stroke and braking force drops sharply.
Manual slack adjusters
Chock the wheels, release the parking brake, build to 90–100 psi, and turn the adjusting nut until the linings touch the drum. Back the nut off about half a turn so the drum turns freely, then recheck push rod travel.
Automatic slack adjusters
Do not hand-adjust them to fix a stroke problem. Automatic adjusters set themselves during normal brake applications; if one is out of adjustment, the cause is usually a worn S-cam bushing, a sticking chamber, or an installation error. Hand adjusting can mask a defect and is permitted only to move a vehicle to a safe place.
Other maintenance items
- Drain all reservoirs daily, especially the wet tank; moisture and oil destroy valves.
- Service the air dryer cartridge on schedule and check its heater in winter.
- Replace linings at ¼ inch thickness (or 1/16 inch above rivets); reject oil-soaked linings.
- Inspect drums for cracks, heat checking, and deep scoring.
- Check compressor belt tension, oil supply, and coolant; a compressor burning oil passes it into the system.
Worked example
A tractor's right steer chamber shows 2⅜ inches of travel on a standard-stroke type 30 chamber. The limit is 2 inches, so the brake is out of adjustment. With manual adjusters, adjust and recheck; with an automatic, find the underlying fault rather than cranking the nut.
Traps
Adjusting with the parking brake applied, or judging adjustment by pedal feel, are the classic errors.