[Woodz428]

Other than the pushrod adjustment mentioned, I would check the return orifice in the master cylinder. Although it was a brand new build, after painting a '70 Mach I I had built about 15 years ago, the brakes had similar issues as yours. All the hydraulics were new other than the time sitting while I prepped and painted the car. Some point in that period, whatever moisture was absorbed from the air settled into the small return orifice within the master cylinder...after a few applications it would gradually get stiffer and then bind. After setting a while they would be O.K. until the next "drive". Although it wasn't completely obstructed, that's why it would release after setting a while, it slowed the return enough that the "drag" heated the fluid up to the point of brake lock. I also saw a similar thing quite often in motorcycles that would set over the winter, I think it was caused by a combination of moisture and the detrioration of the aluminum master enough to allow the contaminants to block or plug the return passage.