1 Is it a dedicated air supply with it's own compressor, to a pressure vessel thus maintaining a constant available pressure
No, it is a bleed from the compressor, variable according to G
2. How is the g actually sensed when in high g turns, and I presume the pressure proportional to the amount of g being applied.
As above, a valve actuated by a weight on a spring. In the Macchi, it was under the left elbow, and to amuse oneself, one could activate the valve and blow the suit up.
3 What pressure would the system be at in say a 6g turn
F*** knows, but the suit gets pretty tight. Makes you grunt and tighten the stomach muscles to fight it, stay conscious and avoid going a little grey. if you don't fight it, the pressure is a bit uncomfortable.
4. When the g comes off is the relief proportional to the lessening of g, and where does the air vent to.
Vents into the cockpit, it releases the pressure proportional to the g.