[mrblaine]

I have not been ignoring you. Just running options through my head.

I was originally thinking bad master, but the symptoms of that would be consistent.

It sound to me as if you have a bad residual pressure valve in the combo block and the shoe springs are eventually overcoming it.

The residual valve is supposed to hold the shoes out near the drum so when the brakes are applied, they have less distance to move to contact the drum.

Not sure how to diagnose this, but it seems that you should be able to pump them back out with the brake pedal and they should hold for awhile.