My truck has had spongy brakes since I bought it. The brakes were spongy while the truck was running, and a little spongy while the truck was shut off. I could also hear a vacuum leak whenever I pushed the brake pedal.

I finally got around to replacing the Booster and figured I should do the master cylinder while I was at it. I bench bled the master cylinder before I installed it. Then bled all the lines starting with the RH rear, LH rear, RABS HCU, RH front and finally LH front. When the truck is off the pedal is ROCK solid, but when the truck is running the pedal goes straight to the floor. The rear brakes are adjusted correctly.

Thoughts, feelings, gestures...?

 

With the engine off and the pedal hard are the brakes working? In as much as being applied?

 

After you installed the m. cylinder, you should have started the bleeding at the m. cyl lines. then at the wheels. Also could have a sticking RABS valve.

 

@artic- Ummm... I think so. It sounded like stuff was moving when I was bleeding the brakes.

@sccamper- If there was still air in the lines or the RABS HCU was sticking wouldn't the pedal be spongy when the truck is off?

 

Almost sounds like a bad booster.

 

Did you bench bleed the master? If not or not sure if ya got all the air out I would start bleeding there as sccamper said. Maybe rent or borrow a pressure bleeder from a parts store.

 

What does the bolded part mean? Are you talking about bench bleeding the master cylinder?

Ichabod said he bench bled the master and bled all the wheels and RABS valve in the correct order. It could be a couple of things:

- bad master (unlikely because you said you have a good pedal with the truck off)
- bad booster (also unlikely)
- incorrect pushrod nut adjustment. When you removed the old booster, did you mark where the nut was on the rod and put it in the same place when you re-installed the booster?
- air in the lines somewhere. Are you sure you got it all out?

 

I had thought about the "incorrect pushrod nut adjustment". Than ruled it out. But it is still a possibility.

 

bet there is still air in the lines.....bleed it again.

perhaps a bad check valve on the vaccum line to the booster?

 

Today I pulled the rear tires and verified brake movement and re-adjusted the shoes to make sure. I also checked the push rod length with the engine running. It was a little short, so I adjusted it out some and that seemed to help a little bit. Anybody else know what the measurement is? Both my manuals say .980 to .995 from the front face of the booster.

I've bled the crap out of the brakes, but brake fluid is cheap so I'll do it again when the wife gets home. But if there was air still in the lines wouldn't the pedal be spongy when the truck was off?

@blue- I pulled the check valve and checked its operation and it's fine.

@pfogle- I have read about "over active vacuum boosters" and I'm wondering if I got one from the parts store.

 

ditto, bleed it again. Check your connections for tightness, make sure you are tightening the bleed screw properly (whatever that means). when the truck is off the pedal does not mean crap. A fully fuinctioning/bled system is always HARD as a rock with no vacuum, but no braking power... you replaced the booster? try replacing the master cylinder. brake parts are cheap, crashing is not. :-)

 

After you installed the master cylinder(all ready bench bleed), and installed and tightened the brake lines; each line should be cracked open one at a time and have someone slowly push the brake pedal. While they hold the pedal, tighten the line. Let off the pedal, crack the line and do it again until no more bubbles. This for me was a lesson earned, not learned.

 

That's what I thought you meant.

I don't know how you "earned" this but it accomplishes nothing except introducing air into the lines. It's useless and wasteful.

 

Does anyone have the answer to Ichabod40's problem ?? I have a 96 F150 with identical problems. I replaced the ABS hyd unit, then bypassed it with a line and fittings,pulled the ABS system fuse,
bled everything multiple times, and still have a pedal that has no braking until it's about on the floor !! HELP !!!!

 

Did you adjust the rear brakes? What other parts have you replaced?