I have a naturally aspirated 2000 S70 with 159,000 miles on it. The car runs almost like new except for a single issue which has haunted it since it rolled off the assembly line.
Every six months or so, it seems the car needs new brakes, front or back. In the last two years, the front calipers have seized twice and the rear once.
The carfax report shows this issue going back almost all the way until the car was new.
The rear brake rotors, pads and calipers, which were replaced a year ago, are already showing signs of excessive corrosion and wear. The car has not been driven excessively as I do not need it to drive to work.
Even though this is New England, last winter had almost no snow, so I do not see road salt as being an issue.
Can anyone tell me what this might be?
Thanks!

You should no longer have a lemon as the calipers have been replaced. Do you park outside? Rotors will get rusty when not used, normally not a problem unless allowed to get to the point of pitting the rotor. The rust is normally ground off with the first application of brakes.

Calipers also rust on the outside, but this is normal for cast iron and should not have an effect on the piston. Slide pins can get dirty and do need cleaning when the pads are changed.

Do you use DOT4+ brake fluid? Does it get flushed every other year whether you drive the car a lot or not? The fluid will get contaminated with water because of the humid climate.

Does your ABS system work normally? When the ABS stops working, too much brake pressure is applied to the rear wheels during normal stopping and aggrevates wear.

What kind of brake pads are you using. The rear should be OEM pads which are reasonably soft and noise free.

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My name is Klaus and I am a V ♂ lv ♂ holic

I couldn't say whether the fluid is DOT 4+ or not. I haven't had it done since I've owned the car. I just keep it topped up.
I do park outside, but the car is used at least every other day.
I assume the ABS is working properly. The light lights up with the others at start up and then goes out, as all the others do, so there aren't any anomalies with that.
I watched the rear brakes being done as I know the mechanic that does them. He used all Volvo parts so I don't have doubts there.

I agree with Chris, it could be a malfunctioning master cylinder, especially since it has been happening since day 1. Ask your mechanic to help with the diagnosis, you can be the helper with pushing the brake pedal.

AND then flush the system. There is something wrong because you should NEVER have to top up brake fluid.
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My name is Klaus and I am a V ♂ lv ♂ holic

When it affects all 4 calipers like that, I wonder it there is an issue with the master cylinder not returning properly. You could jack up all 4 wheels, either all at the same time or one at a time, have an assistant apply the brake pedal and release it, while you spin the wheel and see if the caliper released immediately or seriously drags (or stays locked-up). Then see if it affects just one or two calipers, or all four. Also, does cracking open the bleeder valve allow the caliper to release? If so, then you have an issue up stream and not with the caliper itself.