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Rear Brakes

That is a good question. If you are getting good flow at rear bleed screw and the wheel cylinder piston moves, why aren't the shoes moving. Could be as you suggest the pins are to short, but if they are tight against the shoe slot and the piston cup then they are correct length.
So the another option do the wheels cylinders and master cylinders compliment each other ? Are they matched ?
I re read the 1st post and know this is a 78, but which model truck, Blazer, or Burb ? What size are the rear brakes and what has been changed, Master cyl ? wheel cylinders? Both ?
 
Check valve is good and I have good vacuum at the assist. That is the only piece I haven't swapped. New Master, new wheel cylinders on both, new prop valve and all new lines to the back. They only time I had movement on the cylinder is when I had the shoe off and manually pressed the piston in. Otherwise I haven't seen movement out of them. Tried 2 different kinds of cylinder now. I noticed the ones I got from summit fit 78 blazer rwd, and had a 1 inch bore. Mine being a 4x4 those apparently call for a 15/16 bore. Picked those up last night and put one in, bled, saw no change. This is making me insane. I know it something stupid I missed. But I can't figure this out.
 
Yes very frustrating. What part numbers for master and wheel cylinders ? Smaller cylinder will take less volume to move more. So 15/16 bore is step in the correct direction, but obviously not the problem.
So where is the pressure and flow going if not to the rear wheel cylinders. Iirc you at one time switched the line at the master cyl, have they been put back to original location ??
If so the master cyl may be bypassing internally, I think you have tried 2 masters ? Could 2 masters be bad ? Not likely but not unheard of.
Swelling soft lines can take up the volume of fluid to rears. Wrap your hand around the rear soft line, have someone press the pedal, feel for swelling/ballooning in soft line.
Also try to have helper pump the brake pedal 3-4x while you watch the the rear shoes on one side see if extra pumps make the shoes move.
 
Master Cylinder- Carquest Wearever MCA76162
First set of wheel cylinders from Summit- Dorman Brake Wheel Cylinders W51088 (1 inch bore)
Second set from Autozone - Brakeware 33469 (15/16 Bore)
 
Well all the parts on the truck are correct to work with ea other, on a 4x4 78 blazer, 1.125 bore for MC and 15/16" for wheel cylinder. I am not sure where to go from here.
If you are ok with making a mess. take the cap off mc have someone press the pedal for a normal stop quick stop situation, watch the reservoir should be a fountain from each reservoir. The small side is the rear section. If no fountain from rear side the MC isn't working correctly.
How are you bleeding the brakes ? vacuum bleeder ? If try with a helper pump up brakes, and hold, use a clear tube on bleeder screw, open bleeder screw, close bleeder, pump brakes, do this till no bubbles. Start at farthest brake away from master, right rear then left rear, front right, front left. Keep the master cylinder full, if it goes dry have to start all over again.
Sorry I am not trying to tell you how to do things. This is the way I do it and Always works for me. I have tried pressure bleeders and vacuum bleeders, either end up with a mess, air being drawn in past the threads on the bleeder screw.
double check those soft lines too.
 
Have you blown out/check the rear lines for restrictions? Dont remember, but on the rear drums on these is there to be a residual valve someplace? Is that in place?
 
Iirc the residual back press check valve should be in the master cylinder. It is important to keep the rear side pressurized, but not enough to overcome the return springs
 
Update. New Master cylinder installed. Bled everything again. Brakes felt good, not great but good. No action on the rears after a push. But after a couple pumps they move. Figure all the bleeding must displace that assist in the booster so they need a few pumps. Took it out for a spin. They work. Still feel a bit soft to me but not bad. Every now and then I get a loud sound almost like dragging a heavy table across a gym floor, if I press hard on the brakes for a quick stop. What did I mess up there?
 
if you need to pump either air still in system or rear shoes need to be adjusted tighter. Rear shoes will arch themselves to the drum after some use, and will need adjusting again.
 
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