You aren't lossing much air when you remove the pump, instead you are losing whats in the hose.
Everytime you attach the pump it fills with air, its enough to make a difference though it shouldn't be 50%.
I pump mine up to like 240. If I immediatly take the pump off and put it back on its at like 220 or 210.
The negative chamber on my sid doesn't hold air well, but it seems to have a very small volume as well. so If I have 200 in the negative, and attach the pump it'll be like 160 or something.
For that squishy feeling make sure your airing up the positive chamber, its the top one, higher then the negative. Make sure your sag is around a 1/3 or a quarter of the shock travel. I weigh 280 and I do about 240 in the postive and 200 in the negative, its a very firm ride.