In 2003, I did a disc brake swap from a '73 Duster to my '66 Coronet 500...pretty common then, not so much nowadays! The brakes have always worked, but not to my satisfaction. They've never been really good, just adequate. Here is what I've done since that conversion:
1) Installed a rebuilt power booster and new disc brake dual master cylinder in 2003 when the conversion was done; along with new pads and brake hoses. The calipers from the Duster were in good shape and installed.
2) Five years ago, the LF caliper locked up while I was leaving a car show. Replaced that caliper. Completely replaced the brake fluid at that time with DOT3. Replaced the RF caliper a year later.
3) A few months back, the car took a great amount of effort to stop after just a 15-mile drive! The fluid was low in the front part of the m/cyl, so it was topped off. Later that day, we bled the brakes all the way around. Not much difference. A new master replacement was recommended by a few people, so a new (not r/b) "made in USA" went in.
4) While I was driving the Hot Rod Hundred car run a few weeks back, the brakes once again began to be "flaky". The pedal would go nearly to the floor at times; other times, it was almost normal. I got to a parts store to see if my fluid level was still good and bought brake fluid, just in case. It was normal, but the master cylinder was very hot to the touch...like, just out of an oven hot, but not glowing. The fluid level was normal, however.
So, with this information, I came to the conclusion that perhaps my brake fluid was boiling while I was braking. I'd heard of this, but hadn't experienced it. It was a 220-mile drive, but the moments of using the brakes were few, since we were on State highways for the majority of the run. I've never had a master cylinder be that hot to the touch!
The plan: Check and adjust the adjustment rod in the power booster for proper adjustment; I'm gonna assume for now that it is not. Then, take the car out for a drive around the lake and back home, which is roughly 40 miles at speeds from 35 to 70 mph. If that is not the problem, I'll take the front wheels off and check the discs and the pads for anything out of the ordinary. If things are "normal" again, I guess that's that. If not, I do have new front hoses (still in their bags) that I was going to use on a project that sold several years ago that are the same as the hoses on the Coronet, that I might swap out.
Any ideas? I think I've covered what's up.