Brake Master Cylinder
The master cylinder consists of two pistons that work in tandem to apply pressure to the braking system. These pistons have cup seals that seal fluid in the pressure chamber. As the seal moves past the vent or "compensating" port it begins to create pressure in the pressure chamber through the lines and onto the brake units.
The replenishing port located next to the vent port allows fluid flow to the low pressure side of the piston. As the piston travels through the bore, vacuum is created behind the piston. This flow prevents any vacuum pressure from holding back the piston as it travels forward. A return spring forces the piston back to its resting position as the pedal is released. This action allows the brake fluid to return from the brake lines and piston chamber into the reservoir.
If the brake pedal is over-adjusted or there is debris blocking the vent port, pressurized fluid will not be able to return to the reservoir. This can result in residual pressure in the braking system. This residual pressure will cause brake drag and lock up. Loosening the flare nut at the master cylinder port or at the brake units will relieve this excess pressure, but will not fix the underlying cause.
Primary piston cup seals are located on the pedal side of the master cylinder and the secondary seals toward the front of the vehicle. An O-ring is located at the rear of the primary piston to prevent fluid from leaking past and into the brake booster. When brake fluid leaks past this seal or o-ring it can be seen under the master cylinder on the vacuum booster shell.