Brake Friction Components
The friction side of the brake system is the part techs know best and still lose A5 points on, because the test asks for measurements and causes, not part names. This reference covers disc pads and rotors, drum shoes and drums, and the hardware that holds it together — with the service limits, the right way to describe a pulsation, and the wear patterns that point back to a cause rather than just 'worn out.'
What the friction system does
Disc brakes clamp a rotor between two pads; drum brakes press shoes outward against a drum. Both convert motion into heat, so the A5 questions center on measuring wear and heat-related distortion, and on reading a noise or pulsation back to a cause. The numbers matter: the test expects you to know that parts are serviced against published limits, not by eyeballing them.
Component reference
| Component | Job | Common failure | Symptom / measurement |
|---|---|---|---|
| Disc pads | Clamp the rotor | Worn to indicator/min thickness | Squeal (wear indicator), reduced braking |
| Rotor | Surface for the pads | Thickness variation, below min thickness | Pulsation; measure thickness + variation |
| Lateral runout | (condition) | Excessive wobble | Leads to thickness variation over time |
| Brake shoes | Drum friction | Worn lining, contamination | Reduced braking, fluid-soaked from a leak |
| Brake drum | Drum surface | Out-of-round, over max diameter | Pulsation/noise; measure diameter vs. discard spec |
| Hardware (springs/adjusters/slides) | Locate and return parts | Weak springs, seized adjusters/slides | Drag, uneven wear, noise |
The parts techs mix up
Rotor thickness variation
- Uneven rotor thickness across the swept surface
- Caliper clamps unevenly each revolution → pedal pulsation
- Measured with a micrometer at multiple points around the rotor
- Correct ASE cause language — say this, not 'warped rotor'
Lateral runout
- Side-to-side wobble of the rotor as it spins
- Measured with a dial indicator against the rotor face
- Excessive runout causes uneven pad contact → creates thickness variation over time
- The upstream condition; thickness variation is the downstream result
The call: Pulsation under braking = rotor thickness variation. Runout is what you measure to find why it developed. On the test, always name the condition you can measure and feel — thickness variation — not the folk term 'warped rotor.'
- Minimum thickness vs. machine-to spec. A rotor has a minimum (discard) thickness stamped on it; you can only machine it while it stays above that after the cut. Machining below spec is a fail-the-question answer.
- Tapered pad wear vs. bad pad. Tapered wear — where one end of the pad is thicker than the other — points to seized caliper slide pins, not a worn-out pad. The slide holds one end of the caliper rigid, loading only part of the pad.
- Pad squeal source. A steady squeal is usually the wear indicator doing its job (replace pads); a squeal that changes with braking can be glazing or hardware, not always worn-out pads.
Failure patterns worth memorizing
| Symptom | Real cause | Exam trap / correct call |
|---|---|---|
| Pulsation felt through the pedal under braking | Rotor thickness variation | Say 'thickness variation,' NOT 'warped rotor' — ASE scores the measurable condition |
| Tapered pad wear (one end thicker than the other) | Seized caliper slide pins | Not a bad pad — the seized slide loads only part of the friction surface |
| Brake pull with even hydraulics | Dragging caliper slide or contaminated friction on one side | Isolate mechanically; don't bleed or replace fluid first |
| Drum brake grabs or fades with no hydraulic leak | Contaminated shoes (glazed or fluid-soaked) | Fix the hydraulic source (wheel cylinder) before replacing shoes |
| Return-to-rest drag or uneven pad wear | Weak return springs or seized adjusters/slides | A hardware failure — springs and slides, not the caliper or pad compound |
ASE traps
- Call a pulsation thickness variation, not a warped rotor.
- A rotor can only be machined while it stays above minimum thickness after the cut.
- Fluid-soaked drum shoes point back to a leaking wheel cylinder — fix the hydraulic cause, don’t just replace the shoes. See brake hydraulic components.
Study the full test in the A5 Brakes guide and pair this with the hydraulic and ABS hubs for the rest of the system.