If nothing else, modern master cylinders are notable for their long and relatively trouble-free service lives. The modern master cylinder evolved from the single-piston types popularly introduced in the 1930s to the modern dual-piston, dual braking system cylinders in use today. Master cylinders intended for drum brake use contain one or more residual pressure check valves that maintain a slight pressure in the system. This residual pressure applies tension to the wheel cylinder linkage in the drum brakes and keeps the wheel cylinder piston cups expanded against the cylinder bores. During the 1980s, a quick take-up device was incorporated in some master cylinders to quickly take up the wide brake pad-to-brake rotor clearances existing on some braking systems.  

A master cylinder piston is designed to allow the driver to compensate for a low brake pedal by pumping the brake pedal. The master cylinder bore incorporates a “breather” port located behind the piston cup, which allows brake f...