The rear brake calipers locked on my 85 300D this afternoon when I stopped and went to get a Coke..they literally wouldn't let the car budge an inch!
I walked away in disgust, came back one hour later during which I assume the rear brakes cooled, and they weren't stuck anymore !

I drove off. The brakes felt spongy at first, but okay.
30 minutes later they locked again. I sat there for an hour, let them cool and drove home.


Whats going? Are these calipers rebuildable?

Did both rear calipers lock up at the same time? If so, I would be looking at the master cylinder, not the calipers. Too much of a coincidence.

Apparently this is a common problem as it happened to me and two other posters.
What I would do in your situation is if you can't see a leak coming from the rears that means you may still have seals in them. Park it and replace the master cylinder before you heat the rears up so much the fluid boils and the seals blow. It happened to me on Sandstone Mountain WV while I was happily motoring along thinking 'whoopee I done got me some new stoppers all 'round!' but nooooo. Master tested ok in the shop but it wasn't in the end, sadly. Blew out two new calipers and wore the new pads down to nothing. Oh yeah it toasted my new rotors also.
After I scraped out my shorts I was out for new rotors, calipers, hoses, and pads. That, my friends, was a bummer.
While you're in there check the booster to see if any fluid leaked on the inside. Don't want to mess around with the brakes.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)