friend's 1990 560SEL, he needs to take the brake calipers to a shop for rebuild. He's wondering if he could simply drain all the brake fluid (the brake fluid is old) and let the brake system sit dry with no fluid while the calipers are being rebuilt? I told him I didn't think it would hurt anything......

any input is appreciated.

You have previously inquired about aspects of an ASR system. If this is the same car, CAUTION. The ASR system is complex, and among other things, contains a high pressure accumulator (~2500psi).

That said, the rear calipers can be removed, and the lines capped, not drained.
Disconnect the battery, and do not touch the brake pedal once the system is open.

I would leave the fluid in the lines and flush it after you installed the rebuild calipers.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).