Brake Systems
Hydraulics plus friction plus electronics
Brakes combine Pascal's law, friction, and control electronics. Hydraulic pressure is equal throughout a closed system, and force equals pressure times piston area, so a larger wheel cylinder or caliper piston multiplies force while a larger master cylinder bore reduces pedal travel but raises effort. Pedal ratio adds mechanical advantage at the booster. Most systems are split diagonally or front/rear so a single leak retains partial braking.
Know the hardware and the numbers behind it: rotor minimum thickness is stamped on the casting and is measured away from the rusty lip; lateral runout and disc thickness variation cause pulsation, while stuck slide pins, collapsed hoses, and seized calipers cause drag and uneven pad wear. Pads need bedding, and hardware, shims, and lubricant belong where the OEM says. Drum brakes rely on correct adjustment and free travel.
Fluid is a favourite topic. DOT 3 and DOT 4 are glycol-based and hygroscopic; absorbed moisture lowers the boiling point and invites corrosion, so periodic replacement is normal. DOT 5 is silicone, is not compatible with glycol fluid, and generally should not be used in ABS systems. Bleed in the OEM-specified order, usually starting at the wheel farthest from the master cylinder.
ABS uses wheel speed sensors - inductive two-wire or active three-wire - and a hydraulic control unit. A wheel speed signal dropout disables ABS and often traction control and speedometer functions. After hydraulic work, a scan tool may be needed to bleed the pump.
Trap: a pedal that sinks slowly while held usually indicates an internal master cylinder bypass, not air.
Rule of thumb: diagnose pulsation by measuring runout and thickness variation before replacing parts.