Replaced front brake pads and when the engine is running the brake pedal goes to floor

  • 1 POST
  • 2.0L
  • 4 CYL
  • FWD
  • 91,567 MILES

I have replaced my front brake pads which are disc brakes. After I have done that I got in the vehicle and while it was shut off I pump the brake and the brake pedal got stiff. When I started the vehicle and the engine is running I go to press the brakes again and this time the brake pedal went all the way to the floor. So I did not move the vehicle. What could be causing this? Also I have compressed the piston on the caliper. And had the master cylinder cracked to relieve pressure but put the lid back on before I pull the break or started the engine on the vehicle. When I did that I thought that I would bleed the brakes afterwards and had a friend help me. I bled all four brakes three different times.

Do you
have the same problem?
Monday, October 10th, 2016 AT 12:07 AM

1 Reply

  • 29,775 POSTS

Given the year and mileage, it is likely the master cylinder has been damaged. You must never ever push the brake pedal all the way to the floor unless the master cylinder is less than about a year old. Crud and corrosion build up in the lower halves of the bores where the pistons do not normally travel. Pushing the pedal to the floor runs the lip seals over that crud and can rip them.

The pedal would have been hard to push when there was no power assist, but with the engine running, proper procedure was to push the pedal only halfway to the floor, then release it. Doing that multiple times will run the pistons out of the calipers, and once they take up the slack, the pedal would be normal.

Nothing you said suggests air entered the hydraulic system so there was nothing to gain by bleeding. Another common mistake is inexperienced mechanics and do-it-your-selfers pedal-bleed with a helper, and it is the helper who runs the pedal to the floor. All I have done for over thirty years is gravity-bleeding, especially since I typically do not have a helper around. If there is air in the system, it rarely takes more than ten minutes to get it out. I drive really rusty old stuff so I am commonly replacing steel lines to the rear wheels, and it is not even necessary to bleed at the wheels.

Another thing to consider is if the master cylinder ran empty, and the car has anti-lock brakes, you may need a scanner to command some of the control valves open so the air can be released. Until that is done, the pedal will be soft.

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Monday, October 10th, 2016 AT 1:15 AM

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