Brake fluid is poisonous. It also dissolves paint a lot. See Warning in Section Bleeding the hydraulic brake system.
Although parts and a master cylinder repair kit are available for most models, we still recommend a complete replacement with a new or overhauled master cylinder. It will take you much longer to repair the master cylinder than to replace it, and you won't be able to tell if your master cylinder needs to be overhauled until you remove it. And after that, it may turn out that it is simply impossible to repair it.
Withdrawal
1. The master cylinder is connected to the vacuum booster of the brake system, and both of them are mounted on a bulkhead on the left side of the engine compartment. To remove the master cylinder, disconnect the electrical connector (top arrow), hydraulic brake line fitting (bottom right arrow, other fitting not visible in this photo) and give two nuts securing the master cylinder (bottom left arrow, other nut not visible in this photo).
2. Use the syringe to draw as much liquid out of the reservoir as possible.
3. Place pieces of rags under the line fittings and have caps or plastic bags ready to plug the ends of the tubing after disconnecting.
Brake fluid will damage paint. Cover all parts of the body and try not to spill liquid during this procedure.
4. Loosen the union nuts at the ends of the brake pipes where they connect to the master cylinder. To prevent the flats on these nuts from being torn off, a special split cap should be used («brake») wrench that covers the entire nut.
5. Gently pull the brake pipes out of the master cylinder and plug their ends to prevent dirt from entering and fluid from leaking.
6. Disconnect all electric sockets from the main cylinder, then give nuts of fastening of the main cylinder to the vacuum amplifier of brakes. Pull the cylinder off the studs and pull it out of the engine compartment. Again, be careful not to spill the liquid. Discard the O-ring between the master cylinder and vacuum booster.
This O-ring should always be replaced. A defective ring can cause a vacuum failure, which will reduce the efficiency of the brakes and lead to their intermittent failure.
Bleeding procedure
1. Before installing a new or overhauled master cylinder, it should be blown out on a workbench. It is recommended to install the master cylinder in a vise, as you will have to apply pressure to its piston, and at the same time control the flow from the outlets of the brake pipes. Use a vise with protected jaws and do not over-tighten or the master cylinder body may crack.
2. Insert threaded plugs into the brake line outlets. Tighten them so that no air enters them, but that they are easy to release.
3. Fill reservoir with recommended type of brake fluid (see chapter Car maintenance).
4. Pull out one plug and push the piston assembly inside the master cylinder mirror to remove air from the cylinder. Use a large Phillips screwdriver for this.
5. To prevent air from flowing back into the master cylinder, install and tighten the plug until pressure on the piston assembly is relieved.
6. Repeat the procedure until brake fluid comes out of the brake line outlet without air bubbles. Repeat the procedure for the other plugged brake line outlet. Keep an eye on the brake fluid level in the reservoir to prevent air from entering the system.
7. The bench priming procedure does not use high pressure, so the plugs described above do not have to be installed each time the piston is released. Instead, before releasing the piston, close the hole tightly with your finger to prevent air from entering back into the cylinder. Wait a few seconds for the brake fluid to flow from the reservoir into the piston cylinder, then depress the piston again, removing your finger as fluid flows out. Remember to close the hole with your finger each time you release the piston, and when the procedure for that hole is complete, install the plug and tighten it before moving on to another hole.
Installation
1. Install the master cylinder (along with a new sealing ring) on the vacuum booster studs and tighten the fastening nuts until with your fingers.
2. Connect the brake line fittings to the master cylinder. Since the master cylinder is not yet fully fixed, it can be moved slightly to make it easier to attach the fittings. Do not strip the threads when tightening the fittings.
3. Tighten union connections of brake system and fixing nuts with the demanded effort.
4. Fill the master cylinder reservoir with brake fluid, then bleed the master cylinder (only if it hasn't already been upgraded) and brake system, as described in Section Bleeding the hydraulic brake system.
5. To bleed the cylinder directly on the car, ask an assistant to squeeze the brake pedal several times and then keep it fully depressed. Loosen the union nut to allow air and fluid to escape, then tighten the nut. Repeat this procedure until liquid begins to come out without air bubbles. Thoroughly check the operation of the brakes before starting normal operation of the vehicle.