Brakes pump up, then slowly sink to the floor BUT no leaks???

[Thefarmboy21]

So I recently replaced two hard lines, one rubber hose and rebuilt the driver side brake caliper on my 87....well we bled them a couple times and the brakes pumped up great. Nice and solid (so we thought) until we fired the truck up then BAM right to the floor. Well I drove it a little after the second try an the brakes work and if you pump them a dew times with the truck running, they pump up but your foot slowly just goes toward the floor BUT I'm not losing fluid from the master cyl and u checked for leaks and couldn't find any. So what's the deal??? More air, bad master, bad booster???

 

[firehawk]

Bad cylinder somewhere. Usually the master, but I have seen calipers do this. Sometimes it is several of them. If you have the tools to block off lines and test the master and calipers, that will find the problem; otherwise it is just guess work till the offending part is replaced.

 

[Thefarmboy21]

Well my initial thought was I screwed up the caliper when I rebuilt it but it was working and wasn't leaking. The truck had been sitting 10 years prior to now tho. I guess I'll try to bleed them once more and check for leaks again then try to plug a few lines I guess.

 

[dgr]

Ford has a tsb on this. If you find no leaks, do a search for "zero loss booster" and see if that matches what you have going on.

 

[cpdenton]

Mine did this same thing. I had replaced m/c and a lot of other things on the truck. My symptoms are not completely gone, but I adjusted the rear brakes and the fading pedal got much better. It might still be another problem, but make sure the rear shoes are adjusted well so you can diagnose any other problems. Good luck!

 

[icanfixall]

The test for rear brakes not adjusting properly is to set the parking brake to about half. Then do a slow test drive. If that works then your rear shoes are way out of adjustment. The self adjusters are a known problem with most if not all the ford trucks. Another easy test is to apply the parking brake. If it sets very close to the floor the rear shoes are way out of adjustment. The parking brake peddle should set in the first 1/4 of the peddle reach. Setting in the last 1/4 is a sure sign of bad adjustment of the rear shoes.

 

[Thefarmboy21]

Well I don't think my parking brake even works. Guess I'll find out. Probably should pull the drums and check everything out anyway.

 

[Knuckledragger]

If you have no leaks but have bleed down, it is almost always the master cylinder. The internal seals have gone bad or (more frequently) the cylinder develops a rust spot and the fluid transfers back and forth past the seal, rather than keeping pressure. I vote MC.

How many of us change brake fluid every year or two, as the manual says? When it is dark in the reservoirs, it is time to change.

 

[icanfixall]

I don't recall ever doing a pm with the brake fluid on any of my cars or trucks. When the brake job comes up I usually have the rotors turned and the claipers replaced on the truck. They are so cheap it a shame not to replace them and the hoese. Then all the brake fluid is flushed out bleeding the air from the system. I have either 5 or 6 years on my brake fluid now...