Yesterday was the hottest day so far this year. I took a 100 mile or so trip in the '51 Ford (stock drum brakes, single master cylinder). I noticed as the day went on and it got hotter that my brake pedal got stiffer. Finally, I was on the highway and I noticed the water temp climbing and power dropping off. I pulled on the shoulder and pushed in the clutch and the car stopped itself. The brake pedal was hard and would not move. When I got out, the brake lights were on (pressure switch on master cylinder). I loosened the tube fitting into the master cylinder a turn or so and got a short spray of fluid, tightened it back and checked the brake pedal which was now back to normal. I was 10 miles from home and had not problems the rest of the way. This is a relatively new master cylinder (3 years old) and I flushed the system and added new fluid when I replaced the cylinder. Any ideas?
Brake rod too long will do that. The piston is not coming back far enough to uncover the pressure relief port. A plugged relief port will do the same thing, and silicone based fluid will cause similar problems if the brake rubbers are not made of compatible material. Try adjusting the free play first, that's the most likely fix. And be sure the return spring on the pedal is strong enough to pull the pedal right to the top.
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