Have someone use a pressure tester with a threaded cap adaptor for Volvos (auto shop tool vendors sell them, not Sears however) and it should hold pressure for a long time, left alone. Chances are that it's not so the coolant will then be going somewhere. I'd guess either the inside of your car, remove carpet and plastic cover on left side by gas pedal (possibly either a pipe o-ring or heater core) OR remove spark plugs and see if coolant is accumlating inside a cylinder, not always easily seen btw.