Vanagon GL 86, 2.1, Gas

Last month, the brakes got soft/spongy. They were braking fine, but at
the stop lights they would go down to the floor.
I opened the rear drums and it was time for new shoes and new cylinders.
I changed both, the brakes work great, but at the stop lights again, the
pedal seems to be going down again. Not as bad as before and not all the
time. I opened them again, they looked fine, I readjusted, and it still
does it intermittently.
Any suggestions?

georgios