[davepl]

Residual grease or oil would be my first guess, but sounds like you're fairly sure you've got it all. The rear brakes are on the same line, they just T off after the rubber line, so I would think anything related to the master would affect both sides equally. I was wondering about a residual pressure valve, but again that should affect both sides.

Did you replace the shoes -after- everything was finally clean? If the pads ever got grease or oil on them, no amount of cleaning will likely get it all.

It really sounds like a thermal expandsion issue, given it's OK when cold. If it were me, I'd back the adjuster off on the offending wheel and see if that helps, and if not, I'd swap shoes side to side to see if follows the shoes.