Been working on my brother's 1994 F-250 4x4, Rear Anti-lock brakes.
We are working on brakes. He called and asked about what would cause brake pedal to go to floor. I said any number of things. He narrowed it down to master cyl....that being replaced, still pedal went to floor. I then noticed that when in the truck, you could hear a loud "whoooshing" noise when applying the brakes, leading me to think vacuum booster, so he replaced that. Still no pedal. So now, he tells me the rear brake shoes need replaced along with the brake line to each rear wheel, the wheel cylinders and the spring kits. We do all that, bleed out brakes got the rear brakes all adjusted up...still no pedal. We then commenced to do a total system bleed even the RABS module/unit/ what ever the hell you want to call it. We bled the whole system three times. Nothing. We bled the master cylinder by cracking the brake lines, then doing a bleed of whole system. Still no pedal. What the hell am I doing wrong?
The only thing that I thought may be happening, was one of the brake lines that was replaced was too long. So instead of cutting and double flaring, I coiled the excess line up and attached it to the wheel cylinder. Would the coil of brake line near the wheel trap air to cause spongy, floor dropping pedal?
thanks for any help ,
dano

 

You were right the first time: the extra long line doesn't matter.

Did you bench bleed the MC? Using solid plugs where the lines normally attach?

Roy

 

Roy,
Yes, the mc was bench bled that way.
dano

 

So can you pump the pedal up? Or does it just stay dead?

 

the pedal will pump up when truck is NOT running. Start it up and push on the brake, and the pedal goes to the floor. Here's something else I've never encountered, when pressing on the pedal while its running, it makes a klunking noise as the pedal is dropping to the floor. The klunk seems to shake the truck a little and seems to be coming from the vacuum booster.
dano

 

Oh man.

When the truck is running, and the pedal is to the floor, do the brakes "work" and stop the truck?

No fluid loss, right?

Klunk is new, right?

I bet you returned the old MC and booster for cores?

Roy

 

The only thing I can think of
1 you have a leak
2 you have the wrong master or defective
3 you still have air in the system

Did you kink any lines, did you use the right flair on the lines? If your lines are to long cut them and re flair them to proper size. Should never have pig tails in the lines

 

Roy...
Ummmmm, yeah, both exchanged. Klunk new. Stops truck, no fluid loss.

Tigauto,
Used lines from NAPA, already flared. No kinks, leaks, no runs no hits no errors...cept maybe the pig tail. We had too much line, and didn't want to loop it out where it would snag on anything, so I coiled it up, about a 4'' loop about 1 and 1/2 times in a circle parallel to and above the axle. I really wanted to cut and flare the line but my brother didn't want to as the night was getting late. Another thought on the "pig tail" was that they have about a three loop deal right off the master cylinder, so we thought it would be okay...maybe not, huh?.

 

I have the same problem but without the clunk with my 88. Pumps up when it's off. Goes to the floor and still stops but still too soft when it's running. I bled mine today but still persists. Pedal is firmer than before but still somewhat soft. If your not losing fluid, I'd guess you still have air

 

Have you guys adjusted the brake push rod. Did the new masters come with one or did you use the old ones?

 

Crewcabber,
My brother said the vacuum booster came with the push rod. he didn't adjust it at first because he said it measured the same as the old one. However, as a last ditch effort, we pulled the mc and tweeked it a little, didn't seem to make a difference.
dano

 

You can't base it off the old one. You have aftermarket parts now.

 

My money is on air. You read brake post after brake post, and it is clear guys have problems bleeding these things. The pig tail could complicate the bleed. Maybe a vacuum bleed would be worth a try.

I know your parts are gone as cores, but sometimes is tough situations, I put the old parts back on and see what that changes. Especially if I was not 100% in my diagnosis in the first place.

Roy

 

Gravity bleed them if you have the time.

 

irhunter,
I"m going to bet on the air too. Air, as I have learned, can do funny things.
We are going to do the gravity bleed this weekend. I am also going to suggest he get a shorter line, just to eliminate the chance of the "pigtail" being a culprit.

Crewcabber, the instructions that came with new one said to measure the old and adjust accordingly...tried that, no diff. Tolerances are like crazy, 1/10,000 of an inch for the rod.

Thanks to all. I'm putting my money on air, and getting rid of curly q.
dano