I had no problems with overheated brake fluid on my Lea Francis, 'cos it had rod operated brakes. Those had their own special features. The brake pedal pulled on a rod leading to the front beam axle, then via a swinging link the pull was transmitted to each front wheel. When trying to brake hard at top speed, the cartspring front suspension twisted and allowed the beam axle to move backwards slightly, relative to the chassis. This slackened off the brakes, so the axle would move forward again, reapplying them. Meanwhile, the rod to the rear axle would be doing the same thing, only 180 degrees out of sync with the front. So pulling up from top speed (only about 100 mph) was best done slowly, to avoid shaking the fillings out of my teeth.
Andy