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Brake pedal goes to the floor only when the engine is running

I have a 99’ GMC Yukon Denali. Recently the brake pedal started going to the floor but only when the engine is running. If I turn the vehicle off the pedal will pump up and stay firm. I wedged something between the seat and brake pedal to keep pressure on the system. Looked all around and no leak. The pedal never “fades away” if the engine is off. I figured it was the master cylinder and replaced that. I spent an entire afternoon bench bleeding it until there were no bubbles. I then bled all 4 wheels. Completely replaced all the brake fluid in the system with new brake fluid. There is no air coming out of any bleeder valve at any wheel. When the engine is off I can press the bakes and they are really firm. Go down maybe 1/4 of the way. But if you crank it the pedal just goes to the floor. It’s hard to stop it. Really have to press hard to stop it when the engine is running. It goes basically all the way to the firewall. Not sure what else to try at this point. Any suggestions would be appreciated.

Does this truck have vacuum assisted brakes or hydraulic assisted brakes? If they are hydraulic - run off the power steering pump - the booster may have failed.

If the booster is vacuum assisted, it sound like it has failed although I’ve not had a vacuum booster fail that way.

It’s vacuum assisted. It could be bad. I figured that if that failed it would be hard to push the pedal and it still pumps up when the engine isn’t running. But I guess something on the “other side” of it could have failed.

If the vehicle has ABS, here’s the bleeding procedure when the brake master cylinder is replaced.

1999 YUKON DENALI with ABS

BLEEDING SEQUENCE

Master Cylinder, If Serviced

Right Rear

Left Rear

Right Front

Left Front

Brake Pressure Modulator Valve (BPMV), as equipped

SPECIAL PROCEDURES

When using pressure equipment, use a special tool to depress the metering plunger on the combination valve.

Gravity and vacuum bleeding are not recommended for this system.

With ABS, Brake Pressure Modulator Valve (BPMV) or Hydraulic Control Unit (HCU) have been replaced or are suspected of having trapped air bleed it as follows:

  1. Bleed the wheel brakes using Bleeding Sequence shown above.
  2. Firmly apply the brake pedal and perform three (1993) or four (1996-2000) function tests with the Tech 1 scan tool.
  3. Re-bleed the wheel brakes using Bleeding Sequence shown above.
  4. Inspect brake pedal feel, repeat procedures as needed.

AFTERMARKET APPROACH

OEM Did Not Supply

Tester

It is normal for the brake pedal to drop some when the car is started. You have rear drum brakes, try adjusting them.

The rear drum brakes are adjusted. They are out as far as they can go without causing drag on the rear drums.

I ordered a bi directional scan tool. It took over a week to get here. It didn’t work. Even though I can select the model I have it won’t allow me to auto bleed the ABS module. I’ve also tried using the port on the side of the ABS unit to bleed it. I spent 2 days bleeding that ABS module, then all 4 wheels. I even went and got another master cylinder tried that. Went back through by starting with bench bleeding the master cylinder then the ABS unit then all 4 wheels in the order above. The pedal still goes to the floor when the engine is running.

I’ll have to take it to someone and get them to fix it when I find work again. I’ll park it for now and let let you know what the problem is whenever the world opens back up. It’s too dangerous to drive right now.

Thanks for your help.

They are supposed to cause a little drag on the rear wheels, just enough to feel. You adjust them to a slight drag, make a couple of hard brake applications with the engine running to center the shoes. Check for drag again, if there is none, adjust again.