Drum brake help

[LaOtto70Charger]

I need some help with a drum brake problem.  Basically I have very limited brakes.  I've got '70 with 4 wheel power drums.  I have bled the system several times replaced the master cylinder, but still have bad brakes.  I also noticed recently that with the pedal applied and the engine off the pads only clamp tight for several seconds.  I found this when the vehicle was jacked up.  I had my wife apply the brakes and I would try to turn the drums by hand.  For several seconds I couldn't budge them, but then I'd be able to move the drum rathe easily.  I am able to do that all four corners.  I also haven't seen any fluid leaks.  Any potential things to check next or just keep bleeding the thing?

[Back N Black]

Sounds like you still have air in the system, did you bench bleed the master cylinder? Does the pedal hold firm when you apply pressure or fall to the floor?

[y3chargerrt]

Are you sure the brakes are correctly adjusted?What do you mean by limited brakes? Does the pedel go to the floor or is it just low?

[LaOtto70Charger]

I'm pretty sure the brakes are adjusted correctly.  I threaded out the adjusting screw at each drum until the pads kept the drums from turning freely and than threaded back a part of a turn so they could.  By limited brakes I mean it takes a far longer distance than normal to slow the vehicle.  I've had the car for 15 years now and it never had the greatest distance but not too bad.  Lately it's dangerous to drive.  Even with new brake shoes and a master cylinder.  Also the pedal is low, but it doesn't go to the floor.

I did bench bleed the master cylinder, but probably will do that again.  The problem seems to be the pedal holds firm (low but off the floor), but the shoes slowly loosen at the drums.  That's the part that confuses me.  No pedal position change, but the pressure bleeds away at the drums.  In the past when I had air in the lines, I'd apply the brakes and the pedal goes to the floor.  So I'd pump the brakes until pedal was relatively high.  Now the pedal just goes low but doesn't change as the shoes loosen.

I thought I read the master cylinder is supposed to have residual pressure valve in it due to the design of the rear wheel cylinders.  Is there a way to tell if either of my master cylinders have one?  If it doesn't could that cause this?

Is there anything else in the system that it could be?  (I do have power brakes, but this happens engine off or on.)  I just want to make sure I'm not missing something before I bleed the system from now to eternity.