Q&A for How to Bleed a Master Cylinder

Bench-bleeding a master cylinder involves bleeding it outside the car rather than in the car. The process is straightforward. First, fill both sides of the master cylinder—whether it's dual or single chamber—with brake fluid. Next, pump the brakes to move fluid and air through the open valves at each wheel cylinder. Continue pumping the brakes until only fluid, and no air, is coming through. Finally, tighten all the valves, and you should have pressure on the brakes. This procedure is commonly referred to as a bench bleed.

There may be air in the brake line. Try bleeding the line.

After you change your brake/clutch master cylinder kit or replace the master cylinder assembly, you must bleed air/fluid to prevent 'trapped air' in the system after installing. There would be an 'air pocket' in the line. You have to pump many times by quickly opening and closing the bleed valve so as not to let air come back into the system.

To keep brakes from locking up. ABS will vibrate, keeping brake pads from staying locked. This way, your vehicle will stop smoother without danger of flipping or sliding sideways.

Make sure that you have brake fluid in your master cylinder reservoir. I assume that the master cylinder pistons are okay and pump fluid properly. Too much air in the lines would require more pumping to get fluid through all the way to the bleeder valves. Check that the bleeder valves are not clogged with dirt. Also, check that you do not have any leaks in the system, especially between the master cylinder and the distribution box. You may be pumping fluid out through the leak and not through the bleeder valves. Also, be sure that there are no clogs in the line from dirt or debris falling into the reservoir. Look for damaged brake lines (crushed or kinked) that might be obstructing the flow of fluid.

By keeping the lid on the MC shut tight. The vacuum will hold the fluid while changing from bleed kit to brake line.

Make sure that the brake fluid reservoir is full and then bleed the two rear brake lines, one at a time. Make sure that the reservoir does not empty out during the bleeding process or you will have to bleed out all the lines.

Yes. When you bleed the master cylinder, you should expect fluid coming out of both holes.

If everything is reconnected properly, then you should have no leaks. Try to minimize the time the cover is off. Remove it to fill with brake fluid, then put the lid back on.