Drum brake help

On a non HAMB friendly car im having trouble getting a firm pedal for brakes. When bleeding I get good fluid out of all 4 wheels. When the car is off I can pump the brakes to get a firm pedal but as soon as I start the car the pedal goes all the way to the floor. The car has all 4 new drums, gone through a couple master cylinder, new booster. Can also get the brakes to front wheels and not be able to turn the drums but in the back the drums spin freely. I can figure out what's going on here

Car has 4 wheel drums
Not trying to start trouble with an O/T post just looking for a little help.

Thanks in advance,
david

 

if it is 67 or newer, there is a valve that is for a brake warning light, and if the valve is not "centered" the system won't bleed correctly..

 

New booster,If you tipped the booster up and the cylinder end pushrod was out a little the reaction disc inside could have fallen out of it's cavity.
Pull the rod and seal out,look in the hole or feel around in the bottom of the booster for a small rubber disc.I found this on a mates 4x4 with the same problem only after it happened on my A with late model booster.Worth a try.
Good Luck

 

Make sure that brakes are adjusted properly. Check rubber lines for defects.

 

Thanks
Anyone else??

 

Did u bleed the master cyl on the bench before you put it in? I just bought a new master cyl and it came with two plugs, i kept the plugs for someday troubleshooting brakes. I thought i could isolate the front and rear of the car by pulling one line and putting the plug in and see if my pressure stays or disappears then i would know what direction to go. By cutting the circuit in half makes it easier to troubleshoot. Maybe find some plugs and try to eliminate part of the system. Your trouble may show itself.

 

Yes a couple of times with a few master cylinders. Ill have to try isolating the problem.

Thanks guys
David