Bench Bleeding

Removing air from a replacement master cylinder

Fresh air is normally a good thing, except when it's in your brake lines. Just a bit of the air that keeps us alive can cause a loss of pedal pressure when it travels through your brake hydraulic system resulting in brake failure. Anytime the system is opened, the potential for air to enter is present. Allowing the brake fluid level in the master cylinder to drop, exposing the orifices in the bottom of the reservoir(s) to the air, will allow air to enter the system. The telltale symptom is a soft or spongy brake pedal or the pedal just traveling happily to the floorboard. NEVER drive the vehicle in this condition.

One way to prevent this situation from occurring when changing the master cylinder it to bench-bleed the master before installing it. The process is the same for a single or dual reservoir master and the items needed are easy to obtain. You will need clean DOT 3 brake fluid, a 1/4-inch ID length of clear plastic tubing about 12-inches long, a fitting for the tubing to attach to, a large Phillips head screwdriver or the equivalent and a vise to mount the master. The rest of the process is simple.

Fluid In

The fitting(s) for the hose is installed where the main brake line enters the master cylinder. There are bleeding kits available at auto parts suppliers that have most of the required items included. The kits have a clip that mounts on the lip of the reservoir and holds the tube securely in the reservoir. The end of the tube must be close to the bottom but not touching and then you can add brake fluid. Fill the reservoir about half full and add a little fluid, as necessary. Brake fluid will eat paint so we installed a rag in the vise to deal with any spills or splashes.

Bubbles Out

Use a large Phillips screwdriver to push in the plunger at the back of the master (lower right), forcing the air out of the holes in the bottom of the reservoir. Here you can see massive bubbles exiting the lower chamber. Pressure should be slow and even because pushing too fast will just cause fluid to shoot out of the reservoir and make a mess. Continue pushing until the air bubbles stop coming. At that time you'll see fluid streaming up from the holes in the bottom but no bubbles. If you are doing a dual-reservoir master, repeat the process in the other chamber. Remember, air bubbles are the enemy!

Double-Check

The next step in the process is fluid rising in the clear rubber tube. Once all the bubbles are cleared from the master, pushing on the plunger at the rear will force the fluid into the tube, filling it. At this time you must check the fluid level in the reservoir and add fluid to keep the end of the tube submerged. If you start to suck air, you must start the procedure over again. Once the hose is pumping fluid without air bubbles, and there are no bubbles rising from the bottom of the reservoir, you've successfully bled all the air from the master.

Master Install

Now you can remove the hose, put a plug in the opening(s) where the hose attached for each reservoir and put the top and spring clip (or other type of retainer) on the master. The unit is now ready for installation. Normally if you haven't tampered with any other parts of the system, you can simply install the master in place of the old unit. Start the vehicle and test the brake-pedal pressure. Never drive the car with a low pedal. If the pedal pressure is not satisfactory, continue by bleeding the entire system until you have firm pedal pressure.