a little background to go off of on the truck. it is a 93 7.3 idi with the factor turbo on it. i got it off a good friend of mine and when he got the truck he had problems with the brakes as well. he replaced all pads/shoes, the wheel cylinders, brake booster, master cylinder and bypassed the abs module because he said it was rusted and not working correctly. when i got the truck the brakes worked, nothing great but they worked. drove the truck for a little over a year now and never had any problems. first noticed them going out while i was moving and was pulling a trailer when coming to a stop. the pedal went to the floor when i stepped on it because someone cut me off. i never thought anything of it because the brakes were never that great and it was quite a quick stop. then a while later i was driving to an interview an hour away and halfway there on an interstate i went to press the brakes and they went to the floor. got the truck home safely and went to work. looked around the truck and i didnt see any leaks in the rear anywhere. looked in the front best i could and didnt see anything but its kind of hard to see with how much oil it leaks. all pads are still good. i went ahead and replaced the master cylinder. when i went to bench bleed it i wasnt able to get the port closest to the firewall to bleed at all but the other went fine so when i put it on the truck i tried to bleed it directly at the master cylinder and then it finally bled. i bled them both and there wasnt any air coming out anymore so i tightened everything up. the truck still had no brakes so i bled at all wheels. still nothing. when the truck is off i can pump the pedal a handful of times and get pressure then after 5-10 seconds of holding it will go to the floor. if i pump them and build pressure then turn the truck on they instantly loose all pressure and am nearly unable to get any pressure again. since i dont see any leaks and i bled the crap out of the brakes what could this be?

 

How did the previous owner bypass the RABS valve?

 

he just cut it out and put straight brake line where it was

 

I think you have air in the system. Bleed the brakes again. Start at the master cylinder then RR, LR, RF and LF wheels. Have someone pump the pedal a few of times hold it, crack open the bleeder. A pulse of fluid will come out. Do not wait for the pulse to stop. You have to close the valve mid-stream of the pulse. Check the brake fluid level and add more as needed during the bleed procedure. The pedal should get firm.

 

Last time I had this issue it was the rear wheel cylinders sucking air but they wouldn't leak a drop of fluid. It was around 12$ a side to replace them. I also adjusted the rear brakes while I was in there, she shore does stop now

Another truck I worked on had super touchy yet weak brakes after half a pedal and it was a bad booster that was bad out of the box.

That big fitting that goes on your master and then the line that goes into it also has a oring on it that can leak

 

Sounds like air in the system to me. It's one thing to have a firm pedal and not so good brakes which I'd look at the booster but for that pedal to all of the sudden hit the floor and no visible fluid leaked out it has to have air trapped in it. Like Perk85F250 said start with the RR LR RF LF repeat (farthest away from master to the closest) Open bleeders under pressure and close it under pressure til no air. I'd also double check that bypass hard line and all other brake line connections for tightness. If you have a buddy have him hold the brake pedal down while running to apply maximum pressure to the brakes and see if it's a leaky fitting especially close to the master. I know it's a lot of steps but I've lost my brakes before and almost killed a guy on his bike. It's no joke when it happens.

 

I had a similar problem. I installed three different aftermarket master cylinders. I decided to go to the source and get a Ford part. While I was talking to the parts person an older service tech chimed in about a TSB. Ford replaced the master cylinder with one that had a larger piston bore because of brake bleeding problems. I installed the recommended part and my truck brake problems were fixed. I have looked but can't find the number of the master cylinder I installed, must have put it somewhere safe, you should be able to find the TSB. Good luck

 

I agree, a pedal that goes straight to the floor is usually a bad master cylinder. Air can do it too if the reservoir gets low, usually if it's in a line or wheel cylinder it'll get mushy first.

 

Isn't there a valve below the bed that prevents the rear wheels from locking up when the truck is unloaded? Or maybe that's only on 8500+ lb. GVWR trucks.

I'm pretty sure my truck has one. I saw it when I bled the brakes and wondered if it needed to be included in the bleeding process. I didn't mess with it and my brakes bled fine. Just make sure your e-brake isn't on when you try to bleed your brakes. I know a dummy that did that once.

 

that "valve under the bed" was last used in 86. it was the first version of rear ABS.

 

Thanks for the info.