Had a similar problem on an older car. As above, after use, it was heating up and the piston in the master cylinder wasn't retracting enough to let the fluid in the lines work it's way back to the reservoir. Had to adjust the stroke of the rod.

How much braking are you doing? If you're slowing frequently from high speed, the brakes on these old cars will get hot and fade. While they're hot, it'll feel like you've got no brakes - you can push on the pedal as hard as you can but you just won't stop.
This is one of the main reasons people upgrade to disk brakes - they stay cooler and don't fade as easily.

When cold the brake padal has about 1" of freeplay.
I have cheked the wheels and they are all spinning without drag when cold.

Draining a litle fluid out of the MC seams to have helped but Im not 100% yet.

I checked the rebuild set for the MC towards the old one carefull before I exhanged it and it was a 100% match.
I know there should be a hole where the fluid should return to the reservoir.

I will check if it aligns as it should.

When solved I will let you know.

Thanks all...

Flex lines w/ interiors collapsed sometimes allow fluid through under braking pressure but not return.
Did the car sit for an extended perior of time or anyone ever pinch and hold the flex lines closed w/ grips etc?

Also depending on automatic adjuster style, if the adjusters were mixed up and right/left transposed the brakes may be adjusting tighter w/ each forward braking use unlike the way they're designed to adjust tighter when reversing.

Good luck!