No Brake Pedal Pressure Even After Line Bleeding

Recently in our Ask a Mechanic forums, Carl asked why, after bleeding his brake lines, the brake pedal still sinks to the floor (has no pressure).

The answer is very simple, but many DIY mechanics wouldn't know it. Carl assumes, as would many (including myself), that the problem is the brake master cylinder (or booster) failing to pressurize the brake fluid properly.

That assumption is only partially correct. It's not a broken m/c causing the problem, it's one that was improperly bled (or not bled at all, in this case).

Many are unaware that after draining the brake lines completely, as was the case with Carl's 2000 Chevrolet Blazer - on which some brake lines were replaced - refilling them requires bleeding of both the master cylinder and the lines.

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.

In this case, there was air trapped in the booster itself. 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.