Is it the heat causing the issue or the issue causing the heat? When a hose fails in this matter the main complaint is the wheel getting hot they locking up. Once it cools down everything goes back to normal. You are right both hoses failing is rare, I have only seen it when a car is stored for extended time. I replaced one hose in a Vet for the wheel locking up and a week later the other side failed in the same way and 3 weeks later the rear line started leaking. Luckily I had wrote on the first work order all hoses should be replaced. That was a 1968 that sat for about 10 years and I did it in 1987. Like I said 10 to 15 years they should be replaced to avoid getting little pcs of rubber in your system. I did all of mine a couple of years ago as preventive maintenance.

BTW why are you calipers so expensive? Have you shopped around? Rebuilding them is so cheap and easy. It cost me $10 per caliper. RockAuto had the rebuild kits for my 93 on clearance for $2.

 

Sounds like the same problem I had, chased it around like you. It's going to sound crazy, the little plastic vacuum valve on brake booster I replaced with an Acdelco valve. Sorry don't have part number. No more problems after I changed it. Make sure you use a Acdelco

 

@Pesscomp, I don't believe this is vacuum related as when I unplugged the vacuum or even shut the engine off, the tires were still locked up. I also loosened the bolts between the master cylinder and the booster and put a spacer in there to remove the possibility of incorrect rod adjustment.

This weekend I started the car and let it warm up without driving it and the issue still occurred. I cracked the brake line loose after the master cylinder, brake fluid squirted out and the tires freed up. I ordered ANOTHER new master cylinder yesterday, hopefully I can get it installed this weekend. Stay tuned.
*I also bought new slotted and drilled rotors with pads, but I'm not installing them until later. I want to know exactly what the culprit is.

 

When I removed my master cylinder to replace it, I noticed a rubber grommet(maybe a dust protector?) was pushed into the MC and was preventing the piston from returning to home position. I went ahead and replaced the master cylinder anyway and did not install any kind of grommet between the booster and MC. I drove the car for 30 minutes, the brakes seemed to work great, right when I pulling in my drive they seemed to be getting weak. Unfortunately, I won't be able to test it any further for another two weeks, but I'm thinking the brakes locking up and the brakes going out were not related.

Oh, and I also installed the new slotted/drilled rotor hub assemblies for the front with new pads.

 

It's been awhile, but I decided post a final comment here. Although, after all the above repairs, the brakes seemed to be working better, they were still pretty inconsistent. One day they got super weak, as they did before, but never came back. After driving for about an hour, I felt all the brake system components with my hand and nothing seamed hot. After investigation, I noticed a leak in a brake caliper piston. I replaced both front calipers and the brakes have worked flawlessly ever since. I still don't quite understand what was causing what, but I'm back on the road regardless. Thanks again for the help.