If your shock pump is functioning properly, the pressure you hear escaping is from the hose, which is pressurized along with the fork. As you're unthreading the fitting, the schrader valve closes before the seal on the fitting is broken. Then when you thread the pump back on, the hose refills with air from the fork and since it's such a small volume in the fork, the pressure reads lower. So just pump it up to where you want it, remove the pump, and walk away.