No, remember the back drums were cool. It was the front discs.

 

Good guess! Nope!

 

Nothing exotic beyond a Subaru master with stock Subaru power booster.
BUT your last line is getting very close!!!!! Nothing broken.

 

booster vacuum line loose?

 

Nothing to do with a caliper issue, and yes it does have a line lock, but it wasn't the issue either. Boy this is fun! Should I just spill the answer, or do we want to keep guessing with the bandit being sooooo close?

 

No, that would be too easy! The idle would be too high, and I'd have probably caught it without tearing all my hair out like I did!

 

Master cyl. too full heats up and holds pressure on front calipers. Fixed by lowering fliud level, it happened to me.

 

OK, well I guess I wont let you keep going or make you wait until morning to post what I found.
After a lot of searching the internet with no real direction for a cure, I decided the Subaru master designed for a disc/disc setup must be the source of my problem. I figured I'd just pull off the brake assembly and the master/booster setup, and adapt something else to the Austin that was designed for what I'm running.
I got a drop light and climbed under the dash to begin removing the brake assembly and when I looked up to disconnect the operating rod I noticed that the locknut on the rod adjustment was 1/4" back from where it locks against the clip! I kept looking at it when a light when on in my head. What if the rod had turned out that 1/4" and the pedal no longer had enough freeplay to allow the piston in the master to fully return?
If that happened, then the rear brakes that don't ride on the drums (like front pads do) would not be affected, and the front pads would stay in contact, but with slightly more pressure than usual, and as the rotors began to heat up from this contact they would expand and the brake pressure would increase even more! The increased pressure would then cause the pads to begin braking and that would be the vibration of trying to travel down the road at 60 mph with the front brakes on!
I backed the rod off the 1/4", locked the nut down and pulled the pads to get the glaze off the surface with my belt sander. Reassembled everything and took it for a two hour drive, pulling over several times to check for heat or issues. I could tell right away the difference, but I kept driving to make sure it was really working, and it was!
It's amazing to me that something I easily overlooked could have caused such crazy symptoms. I was ready to blame the master cylinder, line lock, and even the brand new calipers! I'm relieved to not have to replace a bunch of parts and maybe find the cause after a lot of time and money was spent!

 

Well I would have never guessed it. But I'll give my answer anyhow. I was going with the pedal you welded on the otherday being too heavy and applying just enough pedal to drag the pads. I had an issue like that years ago when the aftermarket pedal covers came out. Not the big foot ones, I know those have been around since dirt. I'm mean the ones the ricers started using. The one for the gas was heavy enough to keep the pedal down slightly and the car would cruise at 25 mph with no foot on it. Never idled high though.

BTW thats what you need is a bigfoot gas pedal.

 

You think like a jeenyus! With two guesses the same, you'd think that was it.
At least it wasn't the loose nut behind the steering wheel....this time!