Replacing the master cylinder for your brakes or clutch is a fairly simple task – unbolt the old one, bolt the new one in place, swap the lines and you are done, right? Not exactly. While the actual swap process is easy, there is one more step in the middle that you absolutely must do before you take the new unit to the car: bench bleed the air out.
What Is A Master Cylinder?
The master cylinder is a hydraulic pump. As you pump the pedal, it forces the brake fluid into the calipers/wheel cylinders (or clutch bearing for a clutch master cylinder). This works really well because fluid does not compress. Air, however, compresses quite easily, so any air in the braking system will reduce the hydraulic pressure by large amounts. Even a single air bubble in the brake line can make stopping your vehicle a scary affair. The master cylinder does not automatically bleed the air out of the piston when you fill it, this has to be performed “on the bench”, in other words, out of the vehicle. You can bleed it in the car, but it takes longer and requires two people- one to watch for bubbles and one to push the brake pedal. Bench bleeding is the best option. If you were to try this with the brake lines attached, you would push large amounts of air into the brake lines, and it would take a lot of time and effort to remove all of the air from the master cylinder. You would end up fighting spongy brakes for weeks.
How To Bench Bleed A Master Cylinder
Bench bleeding is done with the master cylinder secured in a vice on a workbench. Some new master cylinders come with a bleeding kit, which consists of two clear flexible tubes, a plastic clip for the lines and two plugs pins. The lines connect to one end of the plug pins, which are inserted into the brake fitting holes in the master cylinder. The lines slide into the clip, which secures the lines to the master.
When placing the lines into the bowls of the master cylinder, make sure they go to the bottom. Every time you actuate the piston, it pulls fluid from the bowl and pushes it out the tubes. When the piston is released, it pulls a vacuum on the tubes. If the ends are exposed, they will suck air, negating the work you are doing.
Fill the reservoir bowls with fluid. The level will drop as you operate the piston. Filling it once to full should suffice.
Using a screwdriver, or ratchet extension, slowly push the plunger on the back of the master cylinder in. Make sure it goes all of the way. Slowly release it, letting it return to the closed position. The plunger is spring loaded, so some effort is required to move the piston. Repeat this action.
As you operate the plunger, the tubes will fill with fluid and air bubbles. As you work through the process you will see how difficult this would be if the brake lines were attached.
Continue to operate the master cylinder until the tubes are free of bubbles. This can take five or more minutes. Tap the side of the master cylinder to help and air pockets release from the side of the housing and out the tubes.
Once all of the air is out, cap off the lines or if unit had plastic caps, reinstall them into the threads.
Now you can take the new master cylinder to the car and install it. Connect the brake lines and brake pedal pushrod. All you need to do now is bleed the rest of the system.
There are master cylinder units available that have bleeder screws on the body, which makes the process easier, but most factory-style master cylinders do not have this feature. While this is a fairly simple process, it can be very frustrating dealing with air bleeding, especially in the main lines that feed the brakes. If you have any concerns or can’t get the brakes to firm up, contact your local NAPA AutoCare Center for assistance.
Check out all the brake system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on what you should know about your brakes and master cylinder, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.