Just a thought....dual drum/drum master cylinders normally have built-in residual valves that maintain a bit of pressure on each of the circuits. If you have it properly plumbed, the rear circuit should retain enough internal pressure (about 10 psi) so that the shoes release but are not pulled too far away from the drums. The front circuit normally holds about 2 psi.
I would:
1. Check to make sure you have the circuits plumbed properly.
2. REBLEED THE SYSTEM....gravity first..and then by conventional pumping into a catch can. Recharging a new system can use up the better part of a quart of fluid to get it completely bled.....and then adjust up shoes to take away any extra travel.
3. If you still are having issues I would add an inline 10 psi residual valve (Summit Racing $16.00) to the rear circuit.
BTW...you need to have your drums on when you are bleeding.
There is a bunch of good brake info at:
http://www.ecihotrodbrakes.com http://www.mpbrakes.com http://classicperform.com Hope this helps.
Dave
