First , I bought this truck from a barn sitting for five years.the trans an rear end needed rebuilding , which I did. while I had the rear end out I also did the disc mod on rear. after 4 months an everything back together, I don't get much fluid out of rear calipers. bleed repeatedly, got some better but not much (spongy pedal). so after reading a lot on here, I bypassed the RWAL an removed it. bleed again , still very spongy. since it was a very old MC and sat for 5 yrs I replaced it, with orellys reman(all that's available). minor improvement not much (very spongy). if you pump it 4 or 5 times rapidly you get a good half pedal, but leaks down. so is MC bad or combination valve? can I bypass combination valve altogether with 4 wheel disc? I have no idea how brakes worked prior to sitting up 5 yrs

 

Did you replace booster?

 

no, however I cant get a firm pedal even with eng off. so I,m thinking booster isn't in play. am I incorrect?

 

I you are just bleeding the rear the "combination valve" is probably moved to the rear and is restricting the line.

If that is the case Try bleeding the front. You should only need to do one.
This should center the piston in the valve. I ha a old chevy that was famous for doing the same thing.

 

ok I ,m heading out to give it a try thanks

 

Bled again , I believe I got combo valve centered,I got a good pedal not running but same ol story once driving

 

I think that you have a bad master cylinder. Unfortunately the state of re-manufactured MC's is weak at best. You may have to Keep trying until you get a good one...Mark

One other thing, did you use the calper with the E-brake? If so, make sure that you have the actuator arm indexed correctly, that is very important. It is also important to use the E-brake, that is how it stays adjusted...

 

I think maybe bad MC too. yes i do have E brake and it is indexed and i do use it.

 

MC on these trucks= Nasty can of worms.

 

FWIW: I did the rear disc conversion on my mess using the Eldorado calipers with E-brake. With the stock/OEM MC, I felt the pedal stroke was too long so, . . . .

- I changed the master-cylinder to one from a 1983 Dodge B350 gasser. It has a cylinder diameter of 1.3125" where as our stock/OEM cylinder is 1.125". That will move more fluid with the same stroke length compared. (Had to narrow the new cylinder's mounting holes to fit our power booster).
- Added a 10lb residual-pressure valve to the rear brake line. This reduces the amount of "Pump-Up required" if you will, with the rear calipers.

Works like a champ!

 

great info thank you

 

I completely forgot about the check valve. I have swapped MCs in other applications to get the internal ones.

 

If/when you get fed up with chasing obsolete master cylinders, booster, etc; I recommend switching to newer hydroboost/MC combo.

 

A very worthwhile swap

 

this seems to be the trend today. hahaha

i would never ever do a rear disc conversion on these trucks, especially with those junk eldorado calipers.

you're sacrificing pad surface and a good e-brake when switching to discs. 3" pads and 1 ton cylinders are great on the backs of these trucks when properly adjusted.