Hydraulic Brake Systems
Hydraulic brakes
Hydraulics work on Pascal's principle: pressure applied to a confined fluid is transmitted equally in all directions, so a larger caliper or wheel-cylinder piston produces greater force. The master cylinder creates pressure; the combination valve proportions front/rear, meters front timing on disc/drum systems, and may hold residual pressure for drum brakes; ABS adds wheel-speed sensors and a hydraulic control unit that can release and reapply pressure.
Diagnose by pedal feel and behaviour. A spongy pedal means air in the system - bleed in the sequence the manufacturer specifies (commonly right rear, left rear, right front, left front). A pedal that slowly sinks under steady pressure means an internal master-cylinder or ABS-unit leak. A low pedal that pumps up suggests misadjusted rear drum brakes or air. Pull on braking suggests a restricted hose or sticking caliper; a flexible hose can fail internally and act as a one-way valve.
Fluid matters. DOT 3 and DOT 4 are glycol-based and absorb moisture, which lowers boiling point; DOT 5 is silicone-based and must not be mixed with glycol fluid. On ABS-equipped vehicles, push the caliper piston back with the bleeder open or the system in service mode so debris is not forced into the HCU.
Worked example. Rotor inspection: measure thickness with a micrometer against the minimum stamped on the rotor, and check lateral runout with a dial indicator on the vehicle (OEM specification, often near 0.05 mm / 0.002 in). Machining below minimum thickness causes heat, warping and pedal pulsation.