Hey guys my 2000 F-150 has a really soft brake pedal. It's been like this since I bought it. It was wrecked in the drivers side cab corner when I bought it. When I first got it it barely had any brake pressure at all! The pedal would go to the floor and just barely stop it! I replaced the master cylinder with one from a junkyard and it has some pressure now, but it still has a soft pedal and goes to the floor! It only seems to have pressure when you push it anywhere from half way to the floor. And it does seem to have better pressure and more pedal when going downhill. The truck had all new brake lines put on it by the previous owner and I've bled all 4 calipers several times and it has fluid at all 4 calipers an no air. Doesnt seem to be leaking brake fluid anywhere.HOWEVER, the ABS light is on and its been on since I bought the truck. Could this be the problem? ShouldI try buying a new master cylinder and see if that solves the problem? Anybody know what's causing this? I appreciate any help or guidance in the right direction you can give me because I'm stumped! Thanks!

 

First thing is physics.
You cannot compress a fluid.
There has to be either air to compress or the master cylinder is bypassing and not building pressure or the actuator rod is not the correct length..
Going down hill with light or no throttle allows the intake manifold to pull higher vacuum on the power cylinder and can affect how it feels with this situation.
ABS normally has no bearing on pedal pressure unless in a traction loss when brakes are applied.
ABS is an assistance action only.
When it does not work as in most of the time, you have regular braking until the sensor detects wheel lack of traction different than the other wheels or end.
It appears you have two problems.
The ABS unit may be have air in it that takes a different method of purging.
Good luck.

 

I'll run this by you....Doesn't sound like you have rear drums, so that adjustment is not an issue. No air at any wheel, that is good. An internal leak in the seals in the master cylinder will cause a pedal to sink down as you put pressure on it. I would buy a new master cylinder....they are not much. Be sure to bench bleed it first. On my 98 I had an ABS light from time to time. On mine you ground a wire behind the RS kickpanel and the ABS light will flash a number of times. A code is then assigned to the number of flashes. Mine was the RABS unit that is directly below the master cylinder. I replaced that and all was good, except I had a low pedal....scary low. The RABS unit had a spot to bleed it so I did that and re-bleed the back brakes and all was good after that....Other than that module that the brake line goes thru, as mentioned....nothing really else on the ABS can cause a low pedal. You can OHM a ABS sensor pretty easy and see if it is good. Usually when one is bad you will get nothing for a reading.....on my truck and Mustang IIRC most sensors are in the 1500-2200 range. If you have rear ABS only that sensor is cheap and easy to change. I busted a rear sensor on the rear wheel on my Mustang GT when I re-built the rear end....that little sucker was $85.00. Good luck.....