having done all the typical isolating moves (loosening M/C, cracking bleeders) to verify it's not a hydraulic pressure issue I doubt it is the M/C.

Though I was never a fan of those generic fits all booster setups based on the vette M/C. They tend to be problematic but usually the problem is not enough brakes, not brakes all the time.

Did you clean and lube the rails the calipers ride on? Are there any worn spots in the guide ways that could be hanging them up? there was a thread on that the other day, with pics.

Found it

It could be rust in the bores of the calipers not letting the pistons come back any, I suppose you could look at that.