Very little pressure to rear brakes.

[thirsty]

It could also be the design differential front/rear pressure ratio of the proportioning valve.  My '87 was built with JB5 brakes.  Braking was so poor that you would pray you were going to stop in time with the pedal nearly mashed through the floorboard!  Very scary!  I'm surprised there was never a recall.  The brakes stayed that way until I reconfigured them.  Now, it stops on a dime!

This probably doesn't apply to your truck, but didn't some of these trucks have a load sensing valve that would limit pressure to the rear wheels unless there was a load in the bed?

Is your truck equipped with this?

One other thing that I have seen is a piece of the foil off a container of brake fluid getting into the system.

[greenmonster]

No load valve.I did flush the system when I put the master cylinder on.I don't think foil or anything is in the system because a lot of fluid flows when i bleed them.

[winky]

hmm... these trucks dont have rubber hosing going up to the rear brakes off the axel do they? if they do it could possibly be swelling under load...

[greenmonster]

They do have a rubber hose.I had my son hit the brakes while I held the line and it didn't swell

[winky]

thats crazy... im really curious as to what is causing your problem...

[bd]

     The only other way to test without throwing parts at it is, (you need a gauge that checks line pressure though), start at back on one side and work your way up to the master cylinder, checking each side one by one.  This method is APITA.  But that is the way to determine what exactly is wrong....

Bake made a valid suggestion.  Or, you could bypass the proportioning valve and temporarily plumb the master cylinder directly to the brakes.  You'll have some time and small expense invested in materials (along with bloody knuckles), but will find out immediately if the proportioning valve is the problem.  Try PM'ing Vile and Captkaos to see what they think.  I'm sure they've run into this problem more than once.

[greenmonster]

I'll see if I can get my hands on a pressure gauge,if not I will bypass the proportioning valve and see what happens.

[greenmonster]

Does anyone know an easy way to test a residual pressure valve.

[bd]

I assume you're referring to the valve in the M/C that maintains a few PSI of pressure in the rear brake circuit.  I doubt that you can test it objectively w/o a pressure gauge.   
But subjectively...    you could apply 1-2 PSI of air pressure (about what you could create with your mouth) through a rubber hose (cheapo M/C bleeder kit) into the outlet port of the M/C and see if air bubbles back into the reservoir.   
I'm really not serious that you try this "distasteful" test.      Not even sure if it would work.  Rather, if you already replaced the M/C with no improvement, the residual check valve was replaced as part of it.  What's to gain?

[bake74]

Replaced master cylinder, wheel cylinders,adjusted brakes,and there is more than 1/4 inch of shoes left.It still can't stop the rear at idle.I can stop the rear with the parking brake.

      Big question, did you bench bleed the master cylinder before putting it on ?  If not you might not get brakes, or sometimes they will work and sometimes not.
      When you flushed your system, did you make sure that all lines were clear and you were getting the same amount of fluid out of all 4 lines ?
     1 other thought, the hard rubber lines for the back could have broken down inside causing a restriction or blockage (that is why the question of all lines flushed and the same).
 

     They do have a rubber hose.I had my son hit the brakes while I held the line and it didn't swell

     They should be reinforced rubber, should not swell enough for you to feel unless they are weak.
     1 last question, ( please see #1 on my signature), do you have the right brakes for your truck, and if you did anything with them, are the back on the right way ?

[greenmonster]

Yes I did bench bleed the master and flushed all the lines until i got clean fluid.With the new master the brakes feel the same as the old master.I think I have the right brakes,11 11/32 x 2 3/4
with wheel cylinder bore of 15/16.I hooked up a small pressure gauge (15 psi) to master and there was no residual pressure.I was reading online that a lot of aftermarket masters don't have the valve in them.Could that be the problem.

[bd]

If the residual valve were missing, pumping the brakes 3-4 times would build pressure.  The more you tell us, the more I question the proportioning valve.  At this point, I'd check the pressure out of the M/C, then out of the proportioning valve (both to front and rear), using a fixed application pressure on the brake pedal, and compare the readings.

[greenmonster]

I'll try that,but I'm still trying to find a pressure gauge.

[bd]

http://www.summitracing.com/parts/SSB-A1704/

[greenmonster]

I took the proportioning valve off to clean it out and figured out that the switch for the brake light was gummed up.After cleaning it and putting it back on the light is on.So that as something to do with the problem.How do I reset that,I tried hitting the brakes hard and it didn't work.