Chevrolet Repair: Mystery Brake Problem, chevy lumina apv, 1991 chevy lumina

Question
Hi Wayne,
I am stumped.  I have a 1991 Chevy Lumina APV van and I have been trying unsuccessfully to get air out of the rear brake lines.  I have successfully bench bled the Master Cylinder, until you can't push the piston in more than 1/8 inch.  I have also tried two different new (refurbished) master cylinders and have bled each brake in sequence(rear pass,rear drivers, front pass, front drivers)at least 50 times each.  I have kept the reservoir full at all times during bleeding. The front brakes have air at first, and then I am able to get all the air out of them.  The back brakes are a different story however as I cannot seem to get the air out of the lines. The brake pedal is fairly firm, but the car rolls to a stop when the brakes are slammed on.  I have adjusted the rear brake shoes so there is a bit of drag when rotating the drums. It appears there may be a leak somwhere, but I don't see any damp areas on the line which would identfy a leak.  I have tried everything I know to do.  I have heard that some Master Cylinders can be bad straight from the store, but I have tried two new ones to try to fix the problem.  Do you have any ideas on what could be wrong?   I appreciate your help!!
Thanks,
Rick

Answer
Hey,

 First things first.  Make sure there is NO hydraulic leaks.  If you are adding fluid and adding fluid you have to check and repair leaks.
 Second is to check hydraulic actuators.  Take all the wheels off. Make sure calipers retract and ALSO that slider are not seized.  Take drums off and push back wheel cylinders.  
 Put it all back together and sit in the drivers seat with your foot on the brake firmly with the vehicle off.  Now start it and make sure the pedal drops a little bit and make sure the vacuum booster is working.
 Air can be introduced just about anywhere including through the mastercylinder.  I have had a bunch of reman masters go bad right out of the box.
 make sure that the proportioning valves are working ok as well.  It kinda sounds like you have a hydraulic distribution problem if there are no leaks and no piston problems.  Double check to make sure you have no seized components.  Especially sliders.