Shockpump test

 It's not just the pressures involved, the volume is the reason for why it won't drop as much. Even if I filled the main air chamber with 200 psi the pressure wouldn't drop more than around 10-15 psi. It's not a certain air pressure the pump have to suck in to read, it's a certain AMOUNT of air. On a big volume chamber that certain amount my pump requires is a lower percent than on a small air chamber, like a piggyback.

 Imagine that the pump needs 50 cc of air and the air chamber on the shock is 50 cc as well. Then the pressure would drop to half (50%) when connecting the pump. If the air chamber on the shock is 150 cc (let's say it's 300 psi), the pressure would drop 75 psi (25%).