Servicing and repairing master cylinders

The master cylinder supplies the brake fluid to the wheel cylinders. The size of the master cylinder is smaller than the wheel cylinder, this causes the master cylinder piston to move more than the wheel cylinder pistons but the wheel cylinder pitons have more force acting on the pads/shoes. Most of the new master cylinders have two pistons, this is mainly for safety reasons. It has two lines that either split front and rear or split diagonally so each piston is connected to two wheels so if one system fails there is another system to slow the car down.

If the master cylinder fails it is probably the seals around the pistons. The pistons need to be removed and the cylinder should be checked for corrosion, because brake fluid draws in water and if the vehicle is parked for too long it can cause the master cylinder to corrode. If it is a steel cylinder it can be honed using methalated spirits or kerosene with a honing tool and then it should be cleaned out with brake cleaner. If the seals have been replaced, the cylinder honed or replaced (if it is aluminuim) and it is assembled it should be bench tested using blocked off fittings to close off the openings for the break lines. The cylinder should hold pressure if it doesn't then there is still a problem with it and it will need to be disassembled again to find the problem. The cylinder should be cleaned with brake cleaner if it will not be installed immediately, otherwise the brake fluid will get contaminated with water and that will cause the cylinder to corrode.

Honing

Secondary piston

Primary piston