You don't say which Volvo you're concerned about, but in either 200 or 700/900 you can prevent Master Cylinder Brake Fluid Loss from any open brake line(s) by blocking the brake pedal depressed before you start. It only takes an inch or two on the 240s.

However 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 would never pinch or clamp a brake hose, regardless of who suggests it. Two of the "Big 3" US car companies are on record against it too..