I' about to give up - here's the stats:

1994 F350 7.3 turbo idi (not powerstroke)

Mushy, spongy brakes.

Recently (within 3 months) replaced master cylinder, vacuum pump, RABS valve, front pads, rear pads. Verified no brake fluid leaks. Verified rear wheel cylinders move and front pistons move when replacing pads & shoes. Adjusted front wheel bearings.

Here's what I didn't do - I didn't turn the rotor or drums. I didn't touch the R wheel cylinders or front calipers.

I plugged the both lines from the master cylinder & got a rock solid brake pedal.

Then I reconnected the rear brakes only and still had a good firm pedal.

Then I reconnected the front brakes too, and got my old mushy, spongy brake pedal back. I bled the system and was surprised that no air bubbles came out, even though I had just reconnected the line to the master cylinder! I must have a high spot in the lines that is trapping the air? Could the rubber lines be swelling that much? What else can cause mushy, spongy brakes.

Also, If the brake pedal is pushed harded than ~ 30 lbs, it slowly falls to the floor.

Any suggestions?

Thx

 

Was the brake booster checked? I dont think that your rotors would cause a spongy pedal.

 

I've held off on replacing the brake booster because I can push the pedal to the floor (slowly) w/o too much effort. After three - four pumps, the pedal get hard to push and the stopping power goes away, like the booster is out of vacuum. From what I've read so far, this is normal.

Also, I test drove the rig w/ only the rear brake lines connected (front line plugged) and the brakes were firm then.

I don't get it......

 

Check the flexible lines on the front axle. There should be two. Theres one at each wheel that runs from the frame to the caliper. Sometimes the inner lining breaks, but the outter hose holds. When you step on the pedal, the hose expands rather then push on the piston in the caliper. Have someone pump the brakes while you hold the hose in your hand. If it expands, replace it.

Trevor

 

A booster can never cause a soft pedal, only a hard pedal.

Papawheelie, that isn't normal. You have some brake problems going on there. Sounds like the master cylinder is no good. I would also replace those rubber lines. That shoud solve most of your problem.

 

that sounds like you have a problem in your master cylinder there pal. You plugged the lines up close, so the pedal was firm, but the more line its got the more room your valves have to move and leak internally. definantly replace that sucker, and probly the rubber lines too, just to be safe. also, check all the hard lines for rust and wear.

 

Thanks Guys,

I will check for brake line swelling tomorrow morning. I didn't know you could feel the line swelling when pressurized.

It DOES act like a master cylinder, which is why I replaced it recently. Then when it STILL acted like it was bad, I plugged the outlets and it firmed right up.

 

I had the same problem with a couple costumers trucks, a 1988 f-250 and a 1995 f-350. the trucks exibited a spongy pedal and no mater what we did we could not get a good pedal. We replaced master, rabs valve, tried bleeding in seqiuence then power bleeding, after all that we had the same pedal. After calling around to a coulpe dealers and getting no help I found a TSB on replacing the master with a larger bore master. Bendix and Raybestos both offer them and they have revised part numbers. I believe the master come with a 1,1/18'' bore stock and the revised master has a 1,3/16'' bore.
I found it hard to believe that this would help, but it ended up taking care of the problem in both vechicles.
I hope this will be of some help to you as I know how you are feeling, I wanted to pull my hair out. good luck
P.S. make sure you check adjustment on the pushrod between master and booster, you need a minimum of .030'' clearence when the truck is cold, otherwise the brakes will drag once you drive around and heat up the brakes

 

somebody correct me if I'm wrong here, but if you didn't turn the rotors or drums, you won't get maximum brake performance. If the rotor is severely out of true, you'll know it because of the shudder. But if it's slightly out of true, the caliper will be further "open" during normal driving. When you brake, the caliper has further to go to get to send the pad to the rotor, requiring more pedal.

May not be your main issue.

 

I know on my 94 f-350 i changed the claipers and rotors, made a huge diff in the feel of the peddel. I did not notice the spongey thing till the new parts were put on.

But my peedel would never go down if i held pressure on it. That sounds like the master to me.

 

That's not how disc brakes work. When the brakes are not applied, the pads rest lightly on the rotor. As soon as you hit the brakes, the pads engage.

Drum brakes are different, though. That's why it's important to keep your drum brakes properly adjusted.

 

Thanks for a few more things to think about folks....

I replaced both front brake hoses today with no improvement. I thought they may be swelling, but I guess not.

Soup2nutz, it sounds promising to me that you have had similar issues and found that the larger bore master cylinder fixed it. I may end up trying that too, but my test with the plugged master cylinder that provides a solid pedal makes me feel like the master is OK.

I may try what clipper1 sez next, but the post mad it sound like the spongy pedal came AFTER the new calipers and rotors. Is that true clipper1?

 

I feel for you as well. I have a 91 F-250 4x4 7.3 and am experienceing the same thing. I even bought a BRAND NEW master cylinder with no change in the pedal. It stops so I kinda just learned to deal with it. I will watch the post and see what yall come up with.

 

The pedel was spongy BEFORE the new parts. I just did not know how bad it was till after i put the new parts on.

 

I also tried to isolate the problem by first plugging the master completely, then one side at a time, I was told to do this by the bendix tech line I called as a last resort, pedal was spongy when fronts were hooked up but great when fully plugged or just on the rears. He was also the guy who first told me about the upgrade for the master and told me it was a volume and pressure issue. So I called my parts guy I deal with at dayton ford and asked him if he heard of similar problems, he laughed and said he had and faxed me over a dealer TSB about the problem you stated above with all revised part numbers. I replaced it and that took care of the problem.
by the way the numbers crossover to aftermarket so you can get bendix or raybestos and not get stuck with paying dealer prices.