How to Prepare a New Master Cylinder for Installation

New master cylinders straight from the box cannot be installed without first removing air from inside the master cylinder. There are several ways to accomplish this but only one that works in most all vehicle types.

Things You'll Need

• Line wrench the size of your outlet fittings
• Wooden dowel rod
• Catch container
• Open end box end wrench
• Fender cover

Instructions

Bench bleeding your new master cylinder

1. First, you will need to remove the old master cylinder from the mount in the engine compartment. Use the service manual for your year model to remove the old master cylinder.

2. Once you remove the old master cylinder, you are ready to "bench bleed" the replacement master cylinder. Mount the new part in a vise with the brake fluid reservoir attached. Attach the bleed lines that come with the new part to the outlet ports on the master cylinder.

3. Install the bleed lines so that they are positioned in a clean catch container that has new, clean brake fluid. If you are unsure of the location of the outlet ports on you master cylinder, consult the instructions that come with the new part. They contain a diagram of the master cylinder with the bleed lines positioned properly.

4. Position the catch container so that your bleed lines are below the outlet ports on the master cylinder. During these procedures, the master cylinder and reservoir remain in the vise. Now you will need to fill the reservoir with new, clean brake fluid. Be sure to fill the reservoir to the full line and maintain the full level throughout the next procedure.

5. You will need to use a wooden dowel rod that is smaller than the piston at the end of the master cylinder. Use the dowel rod to push the piston in and then release the dowel pressure on the piston. You will see air bubbles in the catch container which tell you that the air is being released. Repeat this step several times until the air bubbles stop appearing in the catch container. During this step, maintain a full level of brake fluid in the reservoir.

6. After all the air is bled from the master cylinder, remove the bleed lines and cap the outlet ports. Now replace the master cylinder reservoir cap to prevent air from getting back in the cylinder. You may now take the master cylinder out of the vise and install it in the engine compartment. Follow the service manual for your year model vehicle and install the new part.

Tips & Warnings

• Use a smooth motion when applying pressure to the master cylinder piston to prevent any air from getting in the new part.

• Do not use brake fluid from the catch container because it is contaminated.

• Wear safety glasses throughout the procedure to prevent brake fluid from getting in your eyes.

• Use a fender cover to prevent brake fluid from getting on the paint finish. Brake fluid will remove the paint finish and require a paint job.