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Other than the Tahoe Limited Edition, the vehicle was redesigned for the 2000 model year on the new GMT800 platform, still shared with the full-sized pickups and SUV's.

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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?

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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.

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A brake pedal going to the floor could be indicative of having air in the system still. These can be notoriously hard to bleed all of the air out. The air will get trapped in the ABS pump and will give you a dodgy pedal. I have found activating the ABS system will help to shake the air out. There is a special bleed procedure dealers perform with their scan tool in order to activate the pump. You CAN, although I do not recommend, find a place to jam on the brakes to activate the pump and then rebleed.

If you cannot get a pedal at all, make sure you do not have another leak still, some can be harder to find as they will spray higher up or dribble along the pipe and not drip. If you got a refurbished or used M/C I would double check it to make sure its not internally leaking. They can also leak into the booster hiding a leak.

good luck

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