Auto Repair Q&A



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Brake Lines Bled, Still No Pedal Pressure

Master Cylinder

After doing a brake job that involved draining the entire braking system of fluid in order to replace parts like master cylinders or brake lines, many DIY mechanics will find themselves at wit's end because their brake system no longer pressurizes. Even after being bled. Twice.

The assumption at this point is to think that the master cylinder or booster must have failed. That's not entirely correct. What probably happened was that you've failed to bleed the master cylinder when you re-established the system and filled it with fluid.

You can learn how to bleed the master cylinder at this link. The referenced method works if you've removed the m/c from the vehicle. If not, read on.

A failed power brake booster will usually seem to be OVER-charging the brake lines (adding too much pressure) by locking up the brake pedal so it cannot be easily pushed down. This is because the pedal allows the flow of brake fluid into the booster for compression, but if the booster is broken, that flow will be blocked, locking up the pedal.

Brake boosters have a small bleed screw on them, usually towards the front, that can be opened to allow for bleeding in the same way your brake points underneath the vehicle also have bleed screws.

A simple check of your manual will show you where it's located and the proper torques to be used to open and close the bleeder. The procedure is roughly the same as it is for brake points around the car and most would recommend bleeding the m/c first, before doing each of the lines.