She does say that only one wheel is getting hot. However she wrote, The brake was engaged at the very very top of the pedal. I don't see how this symptom can be explained by a sticking caliper. I can be explained by a sticking master cylinder I think.

I'd look into the possibility mentioned of fluid in the booster or a problem in the MC. It could be that all the brakes have some pressure on them due to a problem in the MC but the one getting hot has the least ability to resist the pressure coming from the MC.

Kerry, if one had less ability to ' resist' the pressure coming from the MC would that not result in ' brake steering' .... a pull to one side or the other when applying the brakes ?
And how would that explain one being hotter than the others ? Which to me indicates failure to back off the rotor sufficiently....

I agree that the left front is failing to back off the rotor. The question is why? Is it a bad rotor or a bad MC. I'm saying the pedal behavior indicates it's possibly a bad MC causes the caliper symptom. One caliper is weaker in its 'pull back ability' so the continued pressure from the MC is having a greater effect on the weaker caliper. One way to determine this would be to loosen the line to the front brakes at the MC, let some fluid bleed out and see if the symptom goes away. If it does, it shows the problem to be pressure in the whole system and not just a stuck rotor.

Okay, here is the whole thing. . .

Earlier this year I was experiencing the inability to STAY STOPPED at a stop. The brake pedal would slowly go to the floor and to stay stopped I would have to pump it up. That was diagnosed as a failed MC and I got one from a forum member. I installed it and for the life of my I couldn't get the brakes to bleed right. I came here and learned that I was doing it wrong (even though my way worked for American cars, didn't work for the MB). So, I bled the brakes the correct way and all was good for about a week.
I was driving the car a distance of about 30 miles and about 20 miles of stop and go traviff, the front left caliper wouldn't back off the rotor. The car wouldn't go (braking hard on that wheel) and I limped to a parking lot and did a quick open/close on the bleed screw for that caliper. That fixed it.
I came here and asked and it was diagnosed as EITHER water in the brake fluid that made the fluid boil and unable to reduce pressure OR a bad flexible line.
So, I rebled the entire system with new fluid AND replaced both flex lines for the front. That seemed to fix the problem. Then there was the non charging issue (replaced volt reg, but that isn't the problem, another thread. . .) but the brakes were fine for several months.
A few days ago I was driving the car and the brake thing happened again.
The brake pedal began engaging higher on the pedal until the pedal would no longer go down at all and the frnt left caliper was stuck engaged and the rotor would only turn slightly, making the car unable to move forward at any real speed. By the time I got home, the wheel was smoking.
When went out today to check on the car, the pedal went to the normal braking point, no longer stuck at the top of the travel. So the pressure had reduced itself after sitting for a time.

So, I've replaced the MC, both front flexible lines and rebled all lines with a BIG bottle of fluid (just kept going until I used the whole bottle in order to make sure there was little to none of the old fluid in it).

Car has maybe been driven 200 miles since the MC was replaced (the beginning of this saga) and it doesn't always happen. I can drive around town with lights and stop signs and nothing, or drive three miles and the caliper sticks, no rhyme or reason)

Thanks!

Was it a new MC?

nope. an old one I had on a working car, I think is the one she's talking about.

How clean are your calipers? have you removed the pads, and cleaned out the path they travel on?

So I don't think the MC can be ruled out.

The caliper SEEMS pretty clean, I spray the area with canned air to get gunk out but since I don't put my head under the car (jacks scare me) I can't see if there is junk anywhere.

So, what is my next step. . .

the caliper sounds like it's working ok, I'd actually recommend pulling each caliper, and removing the pads, and getting a hacksaw blade, and some sandpaper, and cleaning out the area where the pads ride, then put a TRACE of caliper grease on the edges of the pads... VERY VERY VERY little grease, just enough to barely coat the edge of the metal portion of the pad. reassemble, and rebleed the lines...

I'd recommend doing what I suggested. Drive it until the caliper seizes and crack the brake line going to the front wheels at the master cylinder. If that releases the caliper it points to the MC as the cause of the problem.

Does anyone have an explanation for the brake being at the top of the pedal when the problem occurs other than an issue with the master cylinder?

There are two pressure releases on the top of the reservoir, could those be bad?
Could those be anything to this problem?

top of pedal to me would just show a fully bled, and decently working MC... locking up the caliper.
I agree, the first thing to do is wait until a seize and then crack the lines to see if it releases...

nah, those are just vents over the low fluid warning floats...

Yes, there is a thread on that.

I believe it said if your Calipers are frozen and you loosen the Fittings that go into the Master Cylinder up at the Master Cylinder and Brake Fluid spurts out you have a problem with the Master Cylinder not relieving the Pressure.
Also after you do the above the prssure on the Caliper Pistons is relieved.

But, this does not occur often as the other causes.