I appreciate everyone's responses very much. I have driven the car a few days since, and the problem has not returned (I am not fool enough to think it's gone for good). I looked a bit into this topic on the web, came up with brake fluid boiling, the opinion that "rotor warping" is a myth (really deposits onto the rotor and metal structure changes that are the problem), etc. I discounted the boiling, since I didn't lose feel on the pedal. I didn't get pulsing in the pedal either. I am inclined to go with sticky parts. I haven't yet seen the procedure for the fronts, though it must be posted also. The caliper looks quite different, and I didn't observe the pins that are obvious on the back calipers. The rotors haven't been turned to my knowledge - I'll look at thickness. Now I did the back for $100, but that was with third party parts. The front looks more like $150 or so. Thanks again guys...

I've had partially stuck calipers that gradually got hot during about 1 hour of driving and started to drag some at that point. Braking was erratic then with some pulsation and the pedal felt hard. All symptoms went away after a thorough (hours) cooling off but came back on the next drive if I went long/far enough. This did not relate to whether I used the brakes a lot or not; evidently it was a slight but steady drag. The loss of braking power and hard pedal when very hot could have been due to the fluid boiling up in the caliper.

It was a non-Volvo on which a mechanic had put rebuilt calipers that were not of dealer origin.

I've driven some cars with various irregular brake symptoms and when I looked the brake fluid was dark, had flakes in it, or looked otherwise contaminated. If you are replacing hydraulic parts and maybe in any case every few years, a thorough bleeding might be a good plan.

If your rotor has been resurfaced it may be close to or less than minimum thickness. That would also make them tend to warp easier. The min thickness is usually stamped on the rotors but may be hard to find if they're rusty.

Those of us who use brakes lightly have been known to receive sticky slides for our reward. Take the caliper off and grease the slides with brake grease.
Sounds like your rotor got a little warm, watch it carefully. When the pads wear out it will be time for new rotors.

Klaus
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98 V70Rawd(108Kmi), 95 854T(88K mi), 75 164E(173K mi)

Sounds like it could possibly be a sticking (stuck) caliper. If you drive it and ever so lightly push on the brake pedal and it pulsates, then I would guess a warped rotor. You can feel it with your foot. The upside is that rotors and pads are relatively inexpensive online. I did mine for just at $100 delivered, and a cake walk to put on. Might be worth pulling the calipers off, cleaning and lubing the appropriate parts, re-assembling and trying again. Should be able to do it in 1 1/2 hours.