John,B10 drum brakes can be a pain in the but at the best of times,as they are super critical on adjustment and lining to dum alignment.Have the linings been radius ground to the drums,if not this could be a cause.
You should also check the wheel cylinders are going in and out correctly,you can do this very gently with the drums off with some one watching the wheel cylinders,but be very carefull not to push too hard(use you hand on the pedal)or you will pop the seals out.
Also remember,drum brakes are designed to "pump up"due to the valving in the master cylinder.
Check these few things,if all is well with the rears,gently test drive it and report back
Cheers