Thanks everyone for your great suggestions. The problem was that the tiny vent hole in the master was plugged with crud. I rebuilt the master and all is well.Thanks again : Bruce

I would look at the flexible rubber hoses in the brake system sometime the close up internally and restrict the return flow of the brake fluid 

Did you do a line flush when you changed the master cylinder? Could have dislodged some old crap out of the lines that blocked that pesky hole.

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Thanks John, and the rest of you guys for helping me in this. I think I;ll pull the master off and go through it again since it hasn't been touched in 4 years.

There is a very small hole between the master cylinder bore and the reservoir that relieves the pressure in the system when the brake pedal is released.  If that hole is plugged, the brakes will not release, or will build up pressure with temperature increase.  It could be plugged with dirt or rust, or can be covered if there is no free play in the pedal.  It is visible with the cap off if the fluid is clean or the reservoir is empty. 

John - "The Hoosier Hurricane"

Brake fluid attracts moisture. Maybe an excessive amount has built up over a period of time and is expanding when the temperature rises.

I’m familiar with that booster, and after 3 years I’m out of suggestions. Except if there is a residual pressure valve (typically 10lbs for drums), that’s somehow holding more pressure. Could be built in to either the firewall master or the booster master. I just don’t remember from when I had that unit in my bird.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

No, no changes, and everything worked perfect for three years. Bruce

The master is totally mechanically separated from the booster (mounted on the left side inner fender). I started doing this last fall when I was putting cars away for the winter. Thanks : Bruce

I’d be looking for a lack of free play between the pedal and the booster, or the booster and the master cylinder. Very unusual from just turning the heat on in the garage unless you set it up at 20 below 0 or something. Maybe something slipped or hung up on the pedal itself?

Another long shot. Some master cylinders have a “plug” in the bore to allow them to work with both power and manual brakes. Maybe too much free play, and the rod slipped out of the bore in the plug and is hitting the edge. Resulting in no free play or even pressure.

I had that happen with too little free play between my hydroboost unit and the master, but that took 10 miles of driving on a 60 degree day.

Even with that booster, it’s possible, but a real long shot. I’d be looking under the dash and the firewall master first.

Edit. One more thought, is there a residual pressure valve in the system that could be hanging up at higher pressure?

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ