Drum Brake Diagnosis and Repair
Core concepts
Drum brakes multiply force through self-energization: the leading (primary) shoe is dragged into the drum, and in a duo-servo design the secondary shoe is applied through the anchor. That multiplication means lining contamination, a seized anchor pin, or stretched return springs change braking force dramatically. Know the components - backing-plate contact pads, hold-down springs, adjuster screw and star wheel (left- and right-hand threads), self-adjuster lever and strut, wheel cylinder with cup seals and dust boots. Know the measurement rules: drums carry a cast-in maximum diameter; measure with a brake micrometer at the wear ridge and replace if beyond maximum. Inspect for out-of-round, bell-mouth, taper, and heat cracks, and never machine past the limit.
Traps
- The short lining goes in the primary (front) position on a leading/trailing set; reversing shoes causes grabby or uneven braking.
- Star wheels are threaded opposite on left and right sides; the wrong one self-adjusts in the wrong direction.
- Many self-adjusters only ratchet when the vehicle rolls backward with shoes contacting the drum; a seized lever is a common cause of long pedal travel.
- Replace springs in axle sets; reused stretched springs cause drag and chatter.
- Grease-soaked lining cannot be cleaned - it must be replaced.
Worked example
A parking brake will not hold and the pedal is low. Measure the drum; if it is within spec, turn the star wheel until the drum locks, then back off until the drum turns with a light drag. Only then adjust the cable at the equalizer.