Soft Brake Pedal only when car is running



I had to park my car last November because the brake pedal was going to the floor and wasn’t safe. I did notice that the pedal is stiff when the car is off, but once I turn it on the pedal goes to the floor. I took the time to bleed the brakes system and the fluid was a brownish color, but it has been nearly 10 years since it had been replaced. Any thoughts on how to fix it?


It sounds like you are in over your head. You need to take it somewhere and get it fixed. Of course the pedal is hard with the engine off, there is no power assist to help push the pedal down.
Probably the seals on the calipers and/or master cylinder have been damaged by the contaminated fluid.


I agree with Tardis. Trying to fix this yourself is way too dangerous. Your symptoms are not indicative of air in the system, and your not having recognized that combined with the overall nature of the question suggests to me that this needs an experienced brake guy.

Remember that of you mess up a tuneup it can ruin your day, but if you mess up brake work it can ruin the rest of your life.



If you can consistently get braking by pumping the brakes, then the problem might be air in the system. If you can’t or if the amount of pumping needed isn’t pretty consistent, it’s probably not air. And actually if air were the problem, I’d expect the pedal to go to the floor even with the engine off – maybe a bit more reluctantly than with the engine on.

If you aren’t losing brake fluid, I’d bet on a problem with the master cylinder or power assist. I’ve never had the latter fail so I don’t know what the symptoms would be.


It’s likely an MC. The seals in the pistons are probably passing fluid.

The power assist only helps apply the brakes. It’s simply a diaphragm pulled by engine vacuum in line with the rod that pushes into the MC when you apply the brakes. If it fails, the assist disappears but the brakes still work. A failed power assist manifests itself as a hard pedal, but does not affect the function of the brakes.

But this repair will need to be diagnosed directly and done properly including flushing of the system. This is too important to be done by a novice.


Take it to a trusted mechanic.