[Jules]

I'm not a fan of pipe clamps myself they cannot do the rubber any good long term by crushing them to nothing ! (never owned a set)

The easiest way to shut off the master cylinder is to use a suitable item like a piece of wood (our spirit level is just the right length) to depress the brake pedel a few inches.

This closes the reservoir valve in the master preventing fluid loss.
Full pedal depression is not necessary and not possible unless both circuits are open.

The last sinking pedal we had was vacuum pipe related !