Brake Systems
Brake system fundamentals
Hydraulic brakes use Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions. The master cylinder creates pressure; combination valves, proportioning valves, and ABS modulate it. Disc brakes use calipers and pads; drum brakes use wheel cylinders, shoes, and return springs. Brake fluid is hygroscopic, so moisture contamination lowers its boiling point and promotes corrosion.
Traps: DOT 5 is silicone-based and must not be mixed with DOT 3, DOT 4, or DOT 5.1 glycol fluids; DOT 5.1 is glycol and is not the same as DOT 5. A pedal that slowly sinks under steady pressure usually indicates an internal master cylinder cup seal leak, not an external leak. A pull during braking often points to a seized caliper, collapsed hose, or contaminated pads. Rotor thickness below specification cannot be corrected by resurfacing. ABS warning lamps require a scan tool; never assume the problem is a wheel speed sensor without checking codes.
Rule of thumb: flush brake fluid every two to three years or per OEM interval. Moisture raises the risk of boil-over and pedal fade, especially during repeated hard stops.
Worked example: The pedal fades during a long downhill grade, then recovers after cooling. This is fluid boil or pad fade. Check fluid condition and moisture, pad compound, and brake usage. If the fluid is dark or has absorbed moisture, flush with the specified fluid and bleed in the OEM sequence.