Back drum brakes do not always work

[dodge freak 2]

Have a bit of a problem on my Mopar. Had my rear end redone a few years ago and install new wheel brake cylinders in back. Front claipers are not even that old. Brake booster and master cylinder has been changed 1 year ago.

At that time I was bleeding all 4 brakes by having a helper pump the brake pedal up ,I open the bleeder screw, helper would push brake pedal down, I close the bleeder screw and helper would pump break pedal back up and we do this a few times for each wheel. I always keep an eye on the brake fluid level and keep it top up.

Well the front went great but the back seem to go fine but after checking the back brakes they were not working. Break pedal was high and firm. So I open both bleeder screws and let them drip for awhile. After a few mins. just the driver side was dripping. I topped the fuild level up 2X and then closed the drivers side screw. The pass. side then was dripping great again. Let that go for awhile like the driver side. Then I close the pass. side bleeder screw and after topping the master cylinder up, recheck the back brakes. They were working  good.

On to today, my exhaust pipe hanger was loose so I jack the back end up and repaired it. For the heck of it, we recheck the back brakes. No action on either side. So I opened the bleeder screws up and let them drip the same way as before. Brakes work again. Will recheck at the end of the week.

Any thoughts on whats happening, I can not understand why pumping the brakes to bleed won't work but letting the bleeder screws drip to bleed does. And then why do they go out again. Car stops fine-3.91 gears help but still, this is no good. I feel like replacing everything, lines, pop. valve, the hose from by the axle this winter but its most likely just 1 item thats bad.

[Chryco Psycho]

if you do not have a 10 lb residual pressure valve in the system it can suck air in past the seals inside the wheel cylinder , make sure you have a RP valve in the system , they are usually behind the brass insert in the master cylinder where the line attaches , if a piece of wire will go in more than 1/4 or so the rp VALVE IS NOT IN THERE

[Aussie Challenger]

  Yes as above what C.P. said, but also check the steel lines to make sure they haven't been flattened especially as you said your exhaust hanger was loose. I have seen a few exhaust pipes hammer the lines closed and also wear them out.
  Sometimes on the flexible hose the inside collapses or perishes restricting the brake fluid.

[dodge freak 2]

A 10 lb residual pressure valve ?? never hear of that.

All I know is I got the master cylinder and booster @ Auto Zone-never again. I am sticking with aftermarket or Napa from now on.

Have to check that out, back brakes are working and it does seem to stop better. Some times you get lucky, could have wreak the darn thing, thanks Auto Zone.