I seriously doubt your front trunk is getting hot enough to thermally expand the brake fluid alone. Did you check bind was only rear brakes? Both sides? It may be as JFB suggested the push rod length tolerance being too tight. Also it was quite common for aftermarket brake master cylinder adapters to have a plastic guide for the push rod, these are known to be too tight a tolerance and bind when they get hot and need reaming out. If rear only I'd also check flexible line routing near the exhaust, presumably you have changed out the old rubber flex lines for braided hose?
Julian