Don’t know what your running for a master cylinder or brake pedal mechanism but I saw a similar problem on my friends 65 C10. The PO of the truck changed it from a stick transmission to an automatic, he also changed the brake pedal mechanism to a single pedal. The push rod for the master cylinder wasn’t adjusted correctly and when the engine compartment and firewall mounted master cylinder warmed up, the push rod would apply the brakes. Once it cooled off, everything went back to normal. We adjusted the push rod so it had an 1/8” gap between the end of the rod and the master cylinder plunger and it’s worked fine since. Hope this helps.


Dan

35 CC Case
38 Chevy Pickup
51 3100 AD
58 3100 Apache Fleetside