It is common for piston and valve seals inside the brake master cylinder to leak after years of service. Still, you can fix the problem by installing new seals, sold at most auto parts stores as minor brake master cylinder kits. This guide is general in nature and will help you replace the seals on most master cylinders. The steps will help you remove and install the new seals and reinstall the master cylinder in your car.
Park your car in a safe place, open the bonnet and locate the master cylinder. You will find the unit on the rear, driver side of the engine compartment and mounted on the brake booster cylinder--a rounded cylinder on the firewall, about 12 inches in diameter.
Disconnect the brake lines from the master cylinder using an adjustable wrench as backup and a tube wrench to loosen the line nuts. This will prevent damage to the brake lines and nuts. Place some shop rags underneath the master cylinder to catch brake fluid from the lines.
Remove the two mounting bolts securing the master cylinder body to the brake booster. Use a wrench or ratchet and socket.
Remove the master cylinder from the engine compartment and set it on the clean surface of a workbench. Once you remove the master cylinder, plug or cover the brake line openings with clean shop rags to avoid brake system contamination.
Empty the brake fluid from the reservoir into a plastic container and remove the reservoir unit from the master cylinder using a screwdriver. Using snap ring pliers, remove the lock ring securing the master cylinder pistons from the rear of the assembly; then remove the retaining ring securing the valve from the top access where the brake fluid reservoir mounts.
Tap the cylinder assembly on the workbench surface to slide the two pistons, seals and valve assemblies out of the cylinder body. At this point, it is very important to keep pistons, springs, retainers and other internal components well organised so that you can reassemble parts in the right order. Compare the seals from the new kit with the ones in the assembly and replace them accordingly. Reinstall the parts into the cylinder body and install the retaining and lock rings. Install the reservoir on the master cylinder and bleed the unit; see the Tips section for more information.
Set the master cylinder in place in the engine compartment. Start the brake line nuts by hand on the master cylinder to avoid damage to the threads, and install the two mounting bolts. Tighten the lines and bolts.
Bleed the brake system according to your car manufacturer's recommendations and add brake fluid as needed. Check the master cylinder for leaks.
Buy the brake master cylinder kit for your particular vehicle before you begin any work on your brake system to assure availability. You can buy a minor brake master cylinder kit at most auto parts stores. To bleed the brake system and master cylinder and locate components in your particular vehicle, consult your vehicle service manual. You can buy one at most auto parts stores or consult one for free at most public libraries.