Braking Systems Diagnosis and Repair
Core concepts
Understand hydraulic braking principles, master cylinder operation, brake fluid specifications and the dangers of contaminated or moisture-laden fluid. Know disc and drum brake construction, pad and shoe wear indicators, rotor thickness and runout measurement, and the correct bedding-in procedure. Cover ABS operation, wheel speed sensors, brake lines and hoses, proportioning valves and parking brake adjustment. Understand bleeding sequences, the difference between gravity, pressure and manual bleeding, and the importance of not mixing fluid types. Diagnose common complaints such as pedal fade, pulling, vibration, noise and a spongy pedal.
Traps
Questions often test whether brake fluid is hygroscopic (it is) and whether DOT 4 can be mixed with DOT 5 silicone fluid (it cannot). Watch for confusion between runout and thickness, and for the assumption that a spongy pedal always means air in the system when it can also indicate a faulty master cylinder or hose expansion.
Worked example
A customer reports vibration when braking. Measure rotor thickness against the minimum specification and check runout with a dial indicator. If runout exceeds specification, machine or replace the rotor. Also inspect wheel bearings and hub faces, because excessive bearing play or debris on the hub can mimic rotor runout.
Rule of thumb
Brakes are safety-critical. Replace in axle sets and always verify pedal feel before releasing the vehicle.