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Why does my brake pedal go to the floor when the engine is running but

Why does my brake pedal go to the floor when the engine is running but when the engine is running, my brake pedal goes to the floor. when engine is off, I can pump up the brakes somewhat. I have changed the master cylinder and there is no fluid leak. Could the problem be with the power assist boost, or is there something else that could be the issue?

Asked by: Guest | Views: 79
Total answers/comments: 1
mcgyver89 [Entry]

"Hi @kyvic,

What is the model year of the vehicle?

I don't know the vehicle but reading online it seems that it may have either a vacuum brake booster or a hydraulic brake booster given what booster part types are available.

If the brake pedal sinks to the floor when the engine is running then there may be a problem with the brake booster. Here’s a video that may be of some help for a vacuum brake booster. At 3:30 minutes into the video it describes why the pedal may go to the floor - faulty poppet valve in the booster.

Here’s a link that details how to test if a vacuum brake booster is working properly.

I don't know about a hydraulic brake booster but here’s the 2007-2009 GMC Yukon Service Repair Manual + Owner’s Manual. I realize that this is out of the model year range that you selected but in the Brakes > Hydraulic Brakes section, it describes the operation and repair of the hydraulic brake booster. Hopefully it may be the same for your year model."