I have a 1995 F350 Powerstroke dually that has a low brake pedal that continues to sink under pressure. So far I have replaced rear shoes and cylinders,frt pads, master cyl. 2 times, engine vacuum pump(it was on the lower half of the spec range), turned rotors and drums, bypassed the rear anti-lock valve thinking it was somehow faulty(made no difference), bled the master, bled the system with at least 2gallons of fluid(furthest wheel first), adjusted the rears from loose to so tight you could smell them and I still have a mushy pedal that must go at least half way to the floor before any braking begins and will continue to sink to the floor. Nothing is leaking, frt calipers are working fine. With the truck off, the pedal will pump up nice and hard and hold it. But with the truck running the first push is excessively far and mushy at best, a pump or two will firm it up a little but if you hold pressure on the pedal it will sink to the floor with the truck running. A friend of a friend suggested the power vacuum booster may be leaking. Any thoughts on that? I would think if you lost vacuum you would lose the power assist on the brakes and have the equivalent of manual brakes, mine sink to the floor. On the other hand it is about the only thing that hasn't been changed yet. There seem to be lot of low and soft pedal complaints on this forum but no one ever posts if and how they resolved the problem. Help, my mechanic friends are not returning my calls any more and my wife is threatening to make me take it to the dealer! Anything but the dealer!

 

Its the booster i have the same problem with my 95 250.so im going to go get a new booster today and change it over.don't go to a dealer they charge to much.

 

I appreciate the response. From other posts I've read the booster should not be the problem. Several posters have said a faulty booster would create a hard pedal but not a sinking pedal. Keep me posted as to you results. Good luck.

 

I had the same problem in my 86 diesel 4x4 lariat down here in Australia and as you, had all the problems with the brake people trying all. I would be sitting at the lights waiting for them to change green with my foot on the brake and the pedal sinking to the floor. Scary..They said my lines were ballooning and changed to Skyjacker stainlees hoses which made no difference. Put in sfter brake pads and ended up putting in a larger diameter booster AND a vacuum tank near the booster for more instant braking. It's been working well for over a year now.

 

you bought 2 new masters cylinders already?

 

you know its strange i have the same problem with my truck too.I hope my new master fixes the problem.

 

At one stage I thought the master cyl cups were leaking as the pedal went sinking without loss of fluid but the brake people (who had my truck for over a month with a number of experiments) didnt think that was the problem. How would I know? I am only a carpenter and timber merchant, they are the brake specialists. It was MY idea to add the vacuum booster, not theirs so we will leave it at.

 

let me know how you make out!I hope you solve the problem.here is a link to read......http://www.babcox.com/editorial/tr/tr502b.htm

 

If you have changed the master cylinder, then it is very likely that the booster is the problem.

 

This is simple.

Install plugs in the master cylinder.
If the pedal sinks, replace it.
If not look farther down stream.
Have you adjusted the push rod properly?

 

vaccum pump on the motor? i had to change mine in my '97

 

Here's the latest. Decided to start at the beginning. Found some inverted flare plugs and disconnected the lines from the MC and plugged the MC. Had a rock hard pedal with the truck off AND with the truck running- pedal stayed up absolutely no sinking. Seems to indicate the MC is OK. Next I reconnected the front line, leaving the rear outlet plugged, and bled at least 6 times on each side. Good pedal with truck off, a little travel with it running, and you can push it a little farther with great effort but it stayed hard and did not sink as before. Pulled the frt wheels and checked for excessive clearasnce betwween the pads and rotors and caliper and pads( mechanic told me on occassion calipers will draw back toofar which means next time they have travel out farther and you get excess pedal travel). All looked good and tight. Hoses seem ok, no bulges or obvious prob. As much as I hated to, I disconn. the frt line, plugged the frt MC port and reconnected the rears. Bled them 6 times each and had a rock hard pedal with truck running, better in fact than the frt. At this point my mech. friend suggested hanging some new flex hoses on the frts. to see if it may firm up a bit. Unfort. these were nonstock items at shops I called and I was a little nervous about messing with the under truck connections from solid to flex lines and really messing something up at this time( hoping to leave on a trip next week). Decided to just reconnect the frt line, bleed it and see what I have. Ran out of time and patience last nite so its #1 on todays list. Will let you know.

 

I have the same problem on my 94 F250 4x4. I have searched and read all I could find. Anyone know the answer for this?? I don't want to buy and change parts if I don't have to.

 

Anyone????

 

Start at the master cylinder and work your way down.