Hi, for some time now the brake pedal on my 99 F150 has been a bit soft, even before I put all new pads and rotors on. I have 4 wheel disc brakes, I have bleed the lines several times because I thought there might be some air in the lines and still the brakes feel very weak to me. The stopping distance when pulling a trailor scares me at times. I have read many other postings in which others have complained about the same thing. The strange thing I have noticed that when I am pulling the trailor is that when I down shift into second gear to let the engine do some of the braking and step on the brakes when the engine RPM's are up about 3 or 4 thousand the brake pedal feels the way I think it should. However, when the engine RPM drops down to idle and you start to come to a stop the pedal feels a bit mushy again. When towing the trailor I get concerned with the stopping distance at times if I don't engine break. Does the vacuum pressure change with the engine RPM? Any idea why I might be seeing this? Could it be a bad master cyclinder or vacuum booster. Thanks, John

 

Check for dry rot or cracking in the brake lines. Also your brake lines can be swelling when you apply the brakes.

 

If you are reving up the engine as you slow down as you describe, sounds like you are having a vacuum issue with the brakes

 

I agree with PatMan - the fact that running higher revs gives you better brake feel indicates that there is an issue with the vacuum booster and/or vacuum line. Look for any degradation of the vacuum hose that could cause small leaks. Hopefully its just the vacuum hose.

Its odd that you describe it as "soft brakes"... if it is a vacuum loss problem then I would imagine the pedal would feel hard. Try this - sometime when you have some room around you... take it up to about 30 MPH and shift into neutral. Then gently apply the brake (with your left foot). Without changing the force on the brake pedal, rev the engine. If the brake pedal drops away this would indicate that it is indeed a vacuum leak somewhere. When you rev the engine it increases the vacuum level and thereby increases the force in your vacuum booster.

 

easy way to check booster is turn off the truck, pump the pedal 3-5 times it should be hard to push after 5 times.if its still spongy theres your problem.