As long as you are NOT losing fluid somewhere, and the fluid level stays at a constant level in the master cylinder ...... the only thing I'm left with is the same problem that I'm having ...... rear ABS valve and NOT the abs module. Maybe this thread will help, especially the stuff at the bottom of the page - https://www.ford-trucks.com/forums/8...t-working.html

 

i had a leak somewhere and couldnt find it...turns out it was the rear drum brake cylinders. the fluid was leaking inside and the old brake dust/rust was soaking it up. not sure if this will help you as i have a 96 f150

 

What do you mean when you say you removed the RABS module? Did you bypass it with a short length of hard line?

 

Lazy K i did remove the RABS valve and replace it with a length of hard line. I am calling the RABS module the part on the frame just behind the drivers front wheel, after looking at the thread steve posted I realize I am calling that part by the wrong name. So the RABS valve is what I have replaced with hard line and it has not fixed the problem. I cant see it having anything to do with the RABS module as no brake lines even go to it? does my truck even have one?

 

well I isolated only the back brakes and the pedal will still slowly sink to the floor. I have bled multiple bottles of brake fluid through the lines in an attempt to lose all the air. The same for isolated front brakes. Does anyone have any ideas as to where the air/leak is coming from? when both front and back lines are plugged for master cylinder the pedal will not compress. Could it be the master cylinder was not properly bled?

 

The block on the frame rail is variously called the RABS control valve and RABS control module. The computer is usually called the RABS ECM.
However you have eliminated the RABS as a problem. A good point from Th but I would think you would have noticed leaking brake fluid by now.
It would seem that you are not getting all the air out. Maybe time to get/make a pressure bleeder. With one of those you can just pressurize the system, open all the bleed screws and purge the system.
If you want to give normal bleeding another go then bleed right rear, left rear, right front and finally left front. Close each bleed screw before pedal is released each time.
Good luck.

 

Well I put in a another new master cylinder, bench bled it perfect, and put it in. I traced all my lines making sure the connections where good etc. I blocked the back brakes off and started bleeding the front. After over an hour of bleeding the pedal still compresses to the floor, but a lot slower then before. On each side the first bleed air will come out, but after that no air will bleed out. I will switch sides and the same thing will happen. I have been going back and forth like this for over an hour. Is air somehow getting in the system? or is this normal for it to bleed this way/take this long? I cant see a 7 feet of line taking this long to completely bleed out?

 

Well I've talked to 3 heavy duty mechanics who combined have over 100 years experience and they cannot figure out my problem.

When the truck is OFF the brakes will bleed so there is no air in them. The pedal is stiff as can be and will not move a millimeter, but as soon as the truck is turned on the pedal sinks to the floor. When the truck is ON and the front is plugged and the back is plugged the pedal will not sink. Unplug 1 of the fittings from the master cylinder and tie in a line, doesn't matter if it is front or back, the pedal will sink to the floor.

On the front all hard lines have been replaced to both calipers as well as the rubber lines have been replaced with braided stainless. When truck is running pedal will sink to the floor but no leaks at any of the fittings are visible, as well as the lines are not bulging. There is no leaks at the calipers, as well as they are relatively new.

Based on the fact that the system would lose pressure when either ONLY the front OR back was tied in and the other plugged led us to believe it was the master cylinder. 2 replacements later and the fact that when both are plugged I cannot see any bubbling in the resevoir from fluid getting past either of the plungers leads us to believe it is not the master cylinder. As well as when BOTH are plugged the pedal does not compress when the truck is ON.

The back has new drums, shoes, cylinders, ebrake cables, and all new brake lines. There is only 1 hard line that has not been replaced and that is the hard line that goes from just behind front of box to over the axle. Because the problem happens to both front and back that is not the problem.

The RABS valve has been removed and hardline installed in its place in an attempt to find the source of the problem. This has not solved the problem, and it still currently out of the truck.

The only thing left is the booster. I don't understand how this could be the source of the problem but it is the only thing left. Is there any way to determine if it is producing too much or too little vacuum? I still dont see how that would effect the system losing pressure. I am completely stumped on this as well as the 3 HD's who are family friends. Has anyone ever had a problem like this before?

 

The problm is not the booster. Hard pedal with engine off, pedal goes to the floor with engine on are classic indications of air in the system.
Make sure that the brake pedal is not sticking and so not returning to the fully up position when bleeding.
May also think about making a pressure bleeder.
You could also try this. With the resevoir full, crack open all the bleed screws, not full but just enough for a steady drip of fluid. Keep topping up the resevoir untill you have run about 1/2qt of fluid through. Tighten bleed screws and try the brakes. Repeat one or two times if necessary.

 

I had identical symptoms after changing a front caliper on my '88 F-250. Tried everything -- new master, new flex lines, flushed about 3 litres of brake fluid through it trying to get the air out.

I gave up. Took it to the local dealership for a diagnosis. $250 later, they told me a good pressure flushing cured it.