Your problem, for this old thread
Is the brake master cylinder shaft and the end plug that surrounds the shaft that goes to the brake booster.
There is a plug at the end of the M/C made of a fiber material (phenolic) that has failed and it swells and squeezes the shaft when the underhood temperature increases,
and the M/C fails to release.
Thus the brakes are locked on until the temperature goes back down.
I rebuild BMW Audi and Porsche parts
Best,
Jay
M6