The brake master cylinder provides hydraulic pressure for the entire brake system. When the brake pedal is depressed, a slide key activates the master cylinder and presses brake fluid to all four brakes at each corner of the vehicle. When the master cylinder requires replacement, it must be removed from the vehicle which means the brake lines are disconnected from it. By bleeding the master cylinder replacement, you may be able to prevent having to bleed air from the entire brake system and prevent bubbles from entering the system.
Open the jaws of a vise wide enough to accommodate the replacement master cylinder. Place the metal master cylinder body in an upright position between the jaws. Tighten them enough to hold the master cylinder in place, but not to damage it.
Remove the cap from the master cylinder reservoir. Fill the reservoir with fresh brake fluid to the reservoir fill mark.
Connect a short hose extension from the master cylinder bleeder kit to one of the brake lines at the base of the master cylinder. Tighten the fitting with a wrench to prevent damage to the fittings. Repeat this step for all remaining components on the master cylinder.
Insert the opposite ends of the previously installed fittings into the master cylinder reservoir and make sure the ends of the lines are submerged in the brake fluid. If necessary, hold the lines in place by using a plastic spigot provided with the bleeder kit.
Press down on the master cylinder plunger at the end of the master cylinder where it meets the engine firewall with the wooden pin or other blunt object. Keep compressing and leave the plunger until the air bubbles coming from the fittings you just installed subside. Keep pushing on the plunger with short, slow strokes, because it’s going to become very difficult to squeeze while the air bubbles are purging. Keep pushing on the plunger until there are no more bubbles coming out of the fittings.
Fill the reservoir to the brim with brake fluid until it reaches the fill mark and replace the cap. Remove the fittings from the base of the master cylinder with a wrench. Leave the master cylinder in a level upright position until you are ready to install it back on the vehicle.
Tips
When installing a purged master cylinder, tighten the mounting bolts on the firewall with hand tightening. This will allow the master cylinder to move slightly, possibly making it easier to thread the brake lines into the fittings at the base of the master cylinder more easily. Once the fittings are tight, tighten the mounting bolts with a wrench.