I participated in the Metro Cruise (crawl, really) in Grand Rapids,MI. yesterday and it was slow, bumper to bumper. Things had a chance to get real warm but i got out before overheating. Once home, slowly eased it to its spot in the garage. Went to apply brake pressure and was able to push pedal to the floor.
My question: With the master cylinder about two inches from the #7 cylinder exhaust tube, could the brake fluid (synthetic, DOT 3) be at a temperature where it is no longer effective?

that's plenty of clearance, I'm thinking it is something else.

it is possible during the crawl with little air flow, but once you got going again, I would find it unlikely, UNLESS you haven't changed it in a really long time and it has a lot of water in it.

Did it pump back right away?

does it work fine right now?

I had a similar problem in a PU once... the check valve on the booster had failed right there and then.

I did get stopped.

A new check valve form the "help" section fixed it.

Check and bleed the brakes is always a good first step. If it is not a check valve, may be booster or master going south.

I waited a few minutes after the no pedal event and it had more pedal when i pumped the brakes again, and a little more later. From the time I first put the car on the road about 9 years ago, I'd have to say it never really felt like it had great pressure. I always expected to feel more of a firm pedal. This not a power brake system.

It's back to normal, for this car anyway.

I checked the clearance on the brake proportioning valve and the exhaust pipe (at the Y adapter where it splits to the muffler and the cut-out) and it's only about a half-inch between them. I had the exhaust pipes re-done last year and didn't notice at the time how close the valve is to the exhaust.
The fluid is not absolutely clear, with no indication of water. It doesn't look dirty, but I can't account for the slight tinge to the fluid. I've had the brakes bled several times, so the fluid in there now is not the original fluid. I have about 8 thousand miles on the car.

it isn't about mileage, it is about time.
the brake system is open to air and allows it to absorbe water over time.

more water, the lower the boiling point.

they make test strips to tell you how bad it is.

I would let it idle in your driveway & let it get good & hot & see if they go bad again which would tell you if it is strickly heat related. then you can see about either insulating what needs to be protected or dig in further for the culprit.

Thanks everyone for the good ideas. Part of what makes this hobby fun is chasing down problems and (hopefully) solving them

I used to sell master cylinder heat shields. I think Mancini still sells them.

If you haven't flushed the brake fluid in a long time then you might do that first. The brake fluid should really be flushed every couple of years anyway just to keep it nice and clean.