Brake Drag and Heat After a Reline
A technician relined the rear brakes on a low-floor transit bus with Bendix ADB22X air disc brakes. After the road test, the left rear rotor measures 480°F while the right rear measures 210°F. The driver reports a pull to the left and a burning smell. The caliper was replaced as an assembly, the pads were installed with new abutment clips, and the guide pins were lubricated. The parking brake is released and the dash indicator is off. The vehicle has an automatic slack adjuster on the rear axle and the pushrod stroke was set to spec after the reline.
1. List the most probable causes of the left rear drag, in order of likelihood, and the inspection step that distinguishes each.
Read the worked answer
Rank by likelihood on an air disc brake after a reline:
- Guide pin / caliper slide binding - most common. Inspect for torn boots, corrosion, or over-greasing that hydraulically locks the pin. Distinguish by removing the caliper and checking free slide by hand.
- Parking brake chamber or spring brake not fully releasing - even with the indicator off. Crack the chamber, check for residual pressure at the release port, and verify pushrod fully retracts.
- Pad binding in the abutment/caliper - pads installed dry or clips mis-seated. Check pad free movement and clip orientation.
- Restricted air exhaust at the relay/ABS modulator - slow release holds the chamber applied. Compare release time left vs. right at the modulator exhaust.
- Rotor/pad contamination or glazing - would not cause 270°F differential alone but can compound.
Start with the guide pins and parking brake release, since those are the two highest-yield checks and both are quick.
2. The guide pins check out and the parking brake releases fully. The pushrod stroke measures 1 3/4 in. on the left and 1 1/2 in. on the right, both within the applied-stroke limit. What else could cause the drag, and how do you prove it?
Read the worked answer
With pins and parking brake cleared, the differential points to unequal apply/release air delivery or a partially restricted release path on the left.
Prove it with a release-time test:
- Install a gauge at the left and right service chambers.
- Apply to 60 psi, then release and time how long each chamber takes to drop below 5 psi.
- A left chamber that lags by more than a fraction of a second indicates a restricted exhaust - often a collapsed or kinked hose, a plugged fitting, or a sticking ABS modulator/relay exhaust.
Also verify:
- Automatic slack adjuster - a slack that does not fully return can hold the pushrod out even when the chamber releases. Check free stroke and return spring.
- Rotor runout and pad knock-back - excessive runout can drag one side.
Replace the restricted component and re-run the release-time test; the rotor temperatures should equalize within ~50°F after a matched road test.