If you were to clamp the supply pipe (suction line) in my mind surely this would make the mechanical lift pump work harder, although it will reduce fuel flow. So you'd expect a higher negative reading not a lower reading nearer to pressure.

Is that correct?

If it doesn't alter the vacuum reading could it mean a leak within the supply line or a weak lift oump?

I realise I'm asking questins and not helping (I can't if I'm honest) but at least I can learn if my thinking is incorrect.