Braking Systems and Hydraulic Service
This domain covers hydraulic braking: fluid, bleeding, friction pairs, and ABS service.
Core rules
- Glycol fluids (DOT 3, 4, 5.1) are hygroscopic; absorbed water lowers the boiling point and gives a lever that fades when hot or feels spongy. Replace on schedule (commonly every two years) and never mix with DOT 5 silicone.
- Bleed from the caliper furthest from the master cylinder, keeping the reservoir above minimum; never let it run dry, especially with ABS.
- Spongy lever = air or a bulging hose; a lever that sinks slowly to the bar under steady pressure = internal master-cylinder seal bypass.
- Pad friction material minimum is typically 1 mm (replace earlier); rotors have a minimum thickness stamped on the carrier and a runout limit, often 0.15–0.3 mm.
- ABS: use the scan tool to cycle the modulator when bleeding, and read wheel-speed sensor faults before touching hydraulics.
- Bed new pads in with progressive stops, not one hard stop.
Worked example
A rider reports the front lever coming back to the bar after a long descent, and the fluid is dark. Reasoning: the lever is firm when cold, so air is not the main issue; heat-related fade with dark fluid indicates water-contaminated fluid boiling in the caliper. Flush and bleed with fresh DOT 4, then check pad and rotor thickness - if the rotor is below minimum, the reduced heat sink will repeat the fade.
Traps that cost marks
- Topping up with DOT 5 or mixing fluid grades.
- Pushing contaminated fluid back into the ABS modulator when retracting pistons - open the bleed nipple instead.
- Judging pad wear by the backing plate rather than measuring friction material.