My brake pedal goes all the way down to the floor. There is resistance with the engine off but almost no resistance with the engine running. However, during the last couple of inches of travel with the engine running, the car stops very well (at low speeds on my driveway). I’ve bled the brakes three times but I cannot fix this problem.
My brake pads are in good condition but perhaps I’ve damaged the seals in the brake master cylinder by pressing the pedal down too far while bleeding? Or perhaps air has entered the ABS system?
Any advice gratefully appreciated.
Sounds like a bad brake booster.
Why do you think the booster (servo) is faulty?
With the engine off, if I press the brake pedal a few times, it still goes down quite a way but it does show a good deal of resistance to my foot pressure. If I then maintain pressure on the pedal, and start the car, the pedal loses almost all resistance and goes down further. That tells me that the booster/servo is assisting my foot pressure to a very significant degree.
If I then take my foot off the pedal, turn the engine off and wait a few minutes, if I press the brake pedal again, it has almost no resistance and goes right down to the floor (demonstrating the booster’s assistance) but if I then take my foot off and press again, the pedal resists (because the booster’s assistance has been used up). That tells me that the booster/servo is holding vacuum.
I’m no expert on these matters but, to me, the brake booster appears to be working.
I’m making some assumptions that I hope won’t turn around and bite me. If you’ve bled the lines properly, air in the system seems unlikely. And if you had damaged the seals in the master cylinder it seems like you’d have a soft pedal all the time. The only factor that changes is that the booster is getting vacuum when the engine is running. A bad booster or one with a faulty check valve can sometimes produce the symptoms you’re describing. I could easily be wrong though.
If the vehicle has rear drum brakes, check if the brake shoes/drums are worn or if the brake shoes aren’t self adjusting properly.
Because when this happens, the brake pedal has to travel further down before the rear brake shoes come in contact with the brake drums.
Dave, you could very well be right. As I say, I’m no great expert on these matters.
Tester, thanks for your input but the car has discs all-round.
Operate the parking brake several times and see if the brake pedal gets higher.
Tester, I’ve just pulled the parking brake on and off several times and it makes no difference.
With the engine running, and the car in drive it will start to move forward and the foot brake brings it to a halt after being pushed downwards 5 cm (2 inches). However, I can then very easily push the pedal all the way to the floor by pushing it another 4 cm. (just over 1.5 inches).
It sounds like the brake master cylinder is leaking internally
Tester, I’ve already tried to remove the brake master cylinder but it seems to be stuck in place on the servo (booster).
I’ve just watched a You-tube video in which, in order to remove a brake master cylinder, somebody had to cut the lugs off with a hacksaw, but still couldn’t make the cylinder rotate even when gripping it with an 18 inch wrench. He had to add another iron bar to the handle of the wrench in order to make the cylinder turn and thus loosen itself.
It looks like a nightmare job to me.
Remove the plastic reservoir from the master cylinder, and smack the master cylinder with BFH to break it free from the brake booster.
You’re replacing the master cylinder anyway.
Yep I alwats say don’t force it use a bigger hammer.
Tester, I see your logic but will that not bend the captive bolts that hold the master cylinder to the servo (booster)?
There’s two studs on the booster for mounting the master cylinder.
It isn’t going to take much of a blow from a hammer to get the master cylinder off the booster.
I agree; positive proof is if you push on the pedal hard and fast it holds okay, but if you push more gently it goes to the floor slowly without making much brake action>>>master cylinder leaks needs replaced.