Sounds like you have rotor run-out, which allows air to be drawn into the system while you drive. If the brake pads/pistons move in & out, as they would with a warped, or "crooked" rotor, this sucks air past the seals on the caliper pistons. You can bleed them 'til the cows come home, and have a good, solid pedal sitting still, but after driving for a while, air is back in the system, and the pedal gets spongy.
This has been an issue with the Corvette disc brakes since '65, until the '84 models.
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Joel Adams
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