88 F350 brakes wont work!!

[94f450sd]

ok this thing is drivening me nuts!!!the front brakes are the only ones working.have replaced master cylinder 5 times.wheel cylinders twice,shoes,hardware,drums and bled it too many times to remember.found a blown line last night and replaced,figured i found the problem,NOPE!!!.without the truck running if i pump the **** out of the pedal i can get a pedal and it will hold pressure.start the truck and pump it i can get a pedal but it sinks to the floor,when it has a good pedal the rear brakes still aint working.

previous owners replaced the E brake cables but i think they used the wrong ones cuz the E brakes dont work either.i can deal with that later.they also bypassed the RABS valve.i have a known good one off a parts truck that im gonna try today.but when we got the truck it had excellent brakes till my buddy swapped wheels and one side came off on the highway,since then i cant get the brakes to work.

any ideas?where can i get backing plates for it cheap?its the only thing i havent replaced yet.

 

[Dsl_Dog_Treat]

Rich,
Have you done a test on the vacuum system?
Might be worth looking into.

Any air hiding in the lines yet?
These C/C's are a mile long when bleeding brake lines.

I had a post awhile back that addresses the vacuum testing.
I'll see if I can dig it back out of the old pages.

Here it is.
Maybe this will rule one more thing out.

 

[Mr_Roboto]

I'll take a stab.

Is it possible that the proportioning valve was bypassed?

Because disk brakes work instantly, and drum brakes have to have enough pressure to overcome the return springs, the proportioning valve holds pressure from the front until the rears have a chance to engage.

Also make sure that the rear brakes are adjusted all the way out, if they are loose the rears will never work. I adjust them till they just start dragging, the back off a few teeth.