I agree that a locally obtained rebuilt is probably your best option.
However I don't agree that you must replace both rear calipers. Especially in this case, where the "failure" is not due to age or wear.

Your rebuilt may or may not come with the "crash guard weight" that is bolted to the back of the caliper. So be prepared to transfer it from the old caliper.
You should also use a "Flare Wrench" on the brake line fitting -- either 7/16" or 11mm will fit. May be available where you buy your caliper, but possibly only in a set of three.

You dont say what model your Volvo is, but in any case, you can prevent Master Cylinder Brake Fluid Loss from any/all open brake line(s) by blocking the brake pedal depressed before you start. It only takes an inch or two on the 240s.

The 700/900 Green Book shows the 700/900 pedal fully depressed by a special tool (easy to replicate with a 2x4, etc.). The book also says to open a bleeder first, and use a tube and container to catch the fluid expelled when the pedal is being depressed.

>>Disconnect the battery to keep brake lights off while the pedal prop is in place.<<

I've been using this on '80s 240s for more than 10 years now—ever since I learned it from an independent Volvo specialist. I made a wooden prop that hooks to the bottom edge of the pedal and bears against the 240 seat adjuster bar. It's handy for checking brake lights too.

--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.