Hello Papasmurf, I am not sure if the brake pedal on the Porsche has an adjustable screw where it connects to the master cylinder, but many (many..) years ago I had the same issue on a Fiat X1/9 after replacing the master cylinder, I adjusted the pedal travel with '0' play.. and found out that after started driving, once the brake fluid got warm it was activating the brake pistons on the calipers..!
After adding a bit of free play to the pedal the brakes operated normally..
Good luck!
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