Auto Repair Q&A



Cooling System










Auto Repair Products

Why Brake Bleeding Isn't Always Enough

Master Cylinder

When doing a major brake job, such as replacing lines, repairing a caliper, and so on, it’s normal procedure to bleed the brakes in order to get the air out of the system and re-pressurize it. Sometimes this doesn’t work, though, and the do-it-yourself mechanic is left frustrated. Why won’t the brakes re-pressurize?

We often then turn to the master cylinder, assuming that it’s failed or isn’t pressurizing the system properly. That’s partially correct, but most often the problem is that the brake master cylinder hasn’t been bled when the rest of the lines were. A master cylinder with air in it will keep the brakes feeling spongy, whereas a failed power brake booster usually “locks up” the brake pedal, not allowing a press easily.

Like the brake lines, calipers, etc., the master cylinder can also get air into its fluid and not work properly. Bleeding is the fix, of course, and while it’s unusual to find a DIY mechanic who’s done this before, once it’s learned, it’s an easy process. The same applies to when the master cylinder or components on the brake system that are near it have been replaced. Or when draining and replacing brake fluid.

There should be a bleed screw at the bottom of the brake booster and/or master cylinder. The bleed screw will most likely have a connector onto which a temporary brake line hose can be attached to direct the flow of fluid into a container. Following the same procedure used to bleed the brakes at the corners (calipers) will usually do the trick with the master cylinder.

It’s most likely that you’ll have to do another brake bleed of the lines after bleeding the master cylinder/booster. This is due to the brake fluid’s direction of flow.

How to Bleed the Master Cylinder

Before bleeding, make sure that the brake fluid reservoir is completely full. There are two ways to bleed a master cylinder (while still mounted on the vehicle), depending on how your vehicle is designed. Some have a bleed screw on the booster which allows you to do the job with just one connection. Some do not, requiring that the brake lines be disconnected and the M/C bled through their connection ports.

If the former, connect a piece of temporary brake line hose to the bleed screw, run it to a jar or container for collecting fluid, and then open the bleed screw. Then push the brake pedal until only brake fluid is coming from the jar. Close the bleed screw with the pedal still pressed down. Refill the reservoir and check your brakes. It may require two or three attempts to get them just right.

If there is no bleed screw, then a more involved process is required. Press the brake pedal down firmly and keep it there. Then remove the brake lines, attempting to catch the fluid pumping out of the M/C to examine it for air bubbles. If this isn’t possible, that’s okay. Keeping the reservoir full and the pedal down, reattach the brake lines one at a time as fluid moves out of them. This will require a lot of fluid and it’s a messy job.

Once the lines are all re-attached, bleed the brakes as you normally would at each caliper/drum. Your M/C and lines should now all be free of air.