Classic Master Cylinder Rebuild

There is no black magic involved in rebuilding a master cylinder. They are simple hydraulic pumps and come in single- and dual-chamber configurations. Many Detroit vehicles equipped with manual or power drum brakes through the mid-Sixties have a single-chamber master. When disc brakes were introduced around that time, dual-chamber master cylinders were required: one chamber for the front brakes and one for the rear.


They normally consist of a small horizontal cylinder within the housing where the fluid pressure is created. The fluid is then forced into the individual chambers and on through the brake lines to the brakes. This master is from a 1965 GM A-body with power drum brakes, and has only one fluid chamber. Rebuild kits for most master cylinder years and models are available from most auto parts retailers and are considerable less expensive than buying a new master.

Water Damage
The fluid plunger, spring, etc. is held into the internal horizontal cylinder by a circular snap ring. The parts are assembled in the cylinder and the ring is then installed in a groove in the housing. The spring holds tension on the plunger and snap ring. Master cylinder leaks are normally caused by corrosion within the horizontal cylinder. Brake fluid is hydrostatic, which means it attracts water. The water in the brake fluid can cause rust pitting inside the fluid cylinder resulting in fluid leaking past the rubber plunger. Once this happens the only cure is a rebuild or master replacement.

Deep Clean
The master has been thoroughly cleaned with brake cleaner and then washed in solvent. It has been blown dry with compressed air and then minutely examined. Here we're looking into the single fluid chamber. Notice the two holes in the bottom: one hole provides pressure from the horizontal cylinder into the chamber, the other pushes fluid to the brake line (or lines in dual-chamber units). Any dirt or other accumulation in either of these holes will result in fluid or pressure restriction to the caliper (in the case of disc brakes) or wheel cylinder (in the case of drum brakes). Rust pitting was found in the horizontal cylinder, which was allowing brake fluid to leak out the rear of the chamber and onto the brake booster. This type of leakage is the most common and normally caused by rust.

Components
Here are the components of this master cylinder. The cap, body and cap retainer at top, the rebuild kit parts at bottom. It's really simple to rebuild a unit like this. If the rust pitting is minimal, like this example, then a light honing in the cylinder and a rebuild kit will normally solve the problem. If the pitting is deep or throughout, the master will have to be bored oversized and a sleeve installed. In that situation, it might be cheaper to buy a rebuilt master rather than trying to fix your original. If the casting number is important for your restoration, then installing a sleeve in the body will be the answer. The rebuild parts are self-explanatory once you remove the originals.