well, there is a round clip on the brake lines of my 745 and 945. perhaps someone else put this on whilst doing the brakes? at any rate, I'm not sure how to use it, but it definitely appears to be a device to squeeze the brake hose (and, yes, I wonder whether it's wise to do so)
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1991 745; 1993 965 -- RIP: 86 764T; 79 262C

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..

what device are you talking about?
You can allways use "Vise-Grips" with protextion as not to cut the hose (two shims of wood or so)
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EJO now a 1993 944T 211K