Master Cylinder Bench Bleeding & Vacuum Bleeders

By Dennis Bandy Copyright ©  

In this article we cover brake fluid contamination, bleeding air from car brakes, bench bleeding the master cylinder and using a vacuum brake bleeder to bleed automotive brakes. Brake fluid absorbs moisture over time; the water that enters the brake fluid reduces the fluid's boiling point. Water mixes with brake fluid, it does not separate like water in fuel or other petroleum products. Therefore just a visual inspection of the brake master cylinder reservoir will not detect water in brake fluid. There are electronic brake fluid testers  and brake fluid test strips available for detecting moisture in brake fluid. 

 

You will do your customer (or yourself) a great service by detecting excessive moisture in their brake fluid and performing a brake system flush. If brake fluid overheats this can result in the loss of braking ability and a lower pedal. By flushing or bleeding the brake fluid in the hydraulic system and replacing the old moisture contaminated fluid, the brake system is restored back to it's original operating characteristics. The color of the brake fluid in the brake master cylinder may indicate the age of the fluid. I recommend a brake fluid flush for a brake system just as I would recommend a cooling system flush for a cooling system or transmission flush when deteriorated, dark or burnt fluid is detected. You know the old saying about an ounce of prevention being worth a pound of cure.

The most common cause of brake fluid contamination next to water in the system is incorrect fluid type added to the system. The obvious tell tale sign is a swollen rubber seal for the brake master cylinder. Power steering fluid added mistakenly to the brake master cylinder fluid is usually the culprit. The packaging of power steering fluid and brake fluid are nearly identical, the same color labels and many times the bottles are the same size and shape. This makes it very easy to add the wrong fluid. Once the brake system has been contaminated by power steering fluid it can distort all the rubber components within the master cylinder, ABS units, brake hoses, proportioning valves, calipers and wheel cylinders. Bleeding the brakes is not the answer in this case, the only sure way to correctly repair a system that has been contaminated in this manner is to replace all the parts that have rubber components.

Rear drum brakes out of adjustment can cause a low pedal as well as air trapped in the brake hydraulic system. Any brake inspection should include checking the thickness of pads, shoes & using a brake rotor micrometer or brake drum micrometer to measure rotors & drums. If equipped with drum brakes, make sure the shoes are adjusted properly. If the brake fluid level has gone too low, air can enter the system. Inspect the system to find the reason for fluid loss. As brake pads wear the fluid will go down gradually to fill the void in the calipers as the pistons extend (this is normal). A leaky master cylinder can sometimes go undetected if the fluid leaks into the power booster (unbolting the master cylinder and looking into the power booster may be required for inspection). 

Rear drum brake wheel cylinders and disc brake calipers are also possible sources of brake fluid leaks, however be sure to inspect the entire system. Air naturally rises to the highest point but can easily be trapped at any point within the brake system. After any hydraulic component in the brake system has been replaced, air will enter the system. Manually bleeding the brake's hydraulic system, gravity bleeding, power bleeding or vacuum bleeding can be performed. Vacuum bleeding can be done inexpensively and with less set up time compared to pressure bleeding. With a vacuum brake bleeder like the Vacula the technician is able to bleed brakes quickly and with less fluid ending up on the floor.

If flushing the system or bleeding air from the brakes the method is the same. Start bleeding fluid using the vacuum brake bleeder from the farthest wheel cylinder or caliper from the master cylinder. Be sure to keep the brake master cylinder full to prevent air from entering the brake's hydraulic system. When the fluid starts coming out without air bubbles and looks more clear like the new fluid your job is nearing completion, this is easy to see when using a vacuum brake bleeder. Test the firmness of the brake pedal and bleed at each wheel once more to insure all the air has been removed.

If replacing the brake master cylinder, bench bleed the master cylinder before installing it. Some brake master cylinders come with a master cylinder bench bleeding kit to make bench bleeding it easier, purchase a bench bleeding kit if the M/C did not come with one. The bench bleeding kit consists of adapter fittings that screw into the master cylinder and tubes that attach to the fittings. The master cylinder bench bleeding kit we sell has aluminum fittings that fit better and are made to last, unlike the plastic fittings included with some master cylinders. Place the brake master cylinder in a vise and attach the brake master cylinder bleeding kit routing the tubes into the master cylinder reservoir. While keeping the tubes submerged in brake fluid, push the master cylinder piston in repeatedly using a screwdriver. 

Air will be seen traveling through the tubes into the reservoir. Keeping the tubes in the fluid prevents air from re-entering the master cylinder when the piston returns. Once there are no more air bubbles going through the tubes, the master cylinder can be installed on the vehicle. **Caution brake fluid can damage paint, if brake fluid gets on the car, remove it quickly with soap and water** After installing the master cylinder on the car an assistant will be needed to pump the brakes. 

 

Have the assistant apply brake pedal pressure while slightly loosening each brake line at the master cylinder to bleed the remaining air from the system. Repeat this process until there is no more air coming out and the pedal feels normal. In some cases it may also necessary to bleed the brakes at the wheels. The brakes can be bled at the wheels with an assistant or use a vacuum brake bleeder. Fluid evacuators also known as fluid extractors can sometimes double as a vacuum brake bleeder. See our auto blog on vacuum brake bleeding for more information.