How to Change a Brake Master Cylinder Without Bleeding the Entire Brake Systemby Thomas West
The brake master cylinder provides hydraulic pressure for the entire braking system. When the brake pedal is depressed, a push rod activates the master cylinder and pushes 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 must be disconnected from it. By bench bleeding the replacement master cylinder, you may be able to prevent having to bleed the air from the entire braking system and prevent air bubbles from entering the system.
Open the jaws of a bench vise wide enough to accommodate the replacement master cylinder. Place the metal body of the master cylinder in an upright position between the jaws. Tighten the jaws just enough to keep the master cylinder firmly in place, but without damaging it.
Remove the master cylinder reservoir cap. Fill the reservoir with fresh brake fluid to the full mark on the reservoir.
Connect a short length of tubing from a master cylinder bench-bleeding kit to one of the brake line fittings at the base of the master cylinder. Tighten the fitting with a tubing wrench to keep from damaging the fitting. Repeat this step for all remaining fittings on the master cylinder.
Insert the opposite ends of the tubing installed previously into the master cylinder reservoir, making sure the ends of the tubes are submerged in brake fluid. If necessary, hold the tubes in place by using the plastic tab that came with the bleeding kit.
Depress the master cylinder plunger, which is found on the end of the master cylinder where it attaches to the engine firewall, using a wooden dowel or any other blunt object. Continue to compress and release the plunger until the air bubbles coming from the tubes you installed earlier have subsided. Continue to push the plunger in short, slow strokes, as it will become increasingly difficult to depress the plunger while the air bubbles are being purged. Continue to push in on the plunger until there are no more air bubbles coming from the tubes.
Top off the brake fluid in the reservoir until it reaches the full mark and replace the cap. Remove the tubes from the base of the master cylinder with a tubing wrench. Leave the master cylinder in a level, upright position until you are ready to install it back into the vehicle.
- When installing a bench-bled master cylinder, tighten the mounting nuts on the firewall only hand tight. This will allow the master cylinder to move slightly, possibly making it easier to thread the metal brake lines into the fittings at the base of the master cylinder easier. Once the brake line fittings are tight, tighten the mounting nuts with a wrench.
Things You'll Need
- Bench vise
- Brake fluid
- Master cylinder bench-bleeding kit
- Tubing wrench set
- Wooden dowel or equivalent