Community Forums:

re: "...Lucid has posted repeatedly on a method to avoid fluid loss when changing calipers-- search it in the archives...."

As noted in that earlier response, Lucid has offered this method (I'll offer it here, not to take credit -- Lucid please take note -- but in case he doesn't join in personally :-).

What he has recommended is to get some weight on top of (and a block underneath) the brake pedal, so it settles about an inch down from fully up. This apparently moves the master cylinder's piston so that fluid can run from the reservoir into the piston (and beyond to the lines), so in effect the lines are stoppered (like an inverted tube) and this minimizes fluid loss.
[If you don't do this, an open line can drain the whole system (i.e., one of the two separate systems, that is) in a few minutes, leaving a big puddle of brake fluid on the ground, and lots of fluid "bleeding" in your future.]
You will lose a little fluid even with this system, though, so you'll still want to stopper the open line for the duration that it remains disconnected, and you must bleed the line nevertheless.