I have not tested an anti G valve for some years now but doubt it would normally see more than about 10psi
There are a variety of sources on the internet...
But from personal experience - yes starts to inflate at approx 2/2.5 G,I used to turn mine off (as pax) on transits and then turn back on when I was expecting High G turns etc.
The 'bob' weight which operates the anti g valve needs to be very free to move during pre start checks as a 'sticky' weight is ultra uncomfortable as alluded to by Wensleydale.
In a manoeuvre, the valve should start functioning at about 2 G . The pressure should be such that at 1½G it is zero and from then on the pressure varies at the rate of 1 p.s.i. per ‘G’. Thus at 2 G , pressure is ½ p.s.i., at 4 G it is 2½ p.s.i. etc.
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G: range 1 to 10
G suit pressure: 0 to 11 psig (PSI Gauge).
Source pressure: 0 to 300 psig (from engine compressor)