How to stop air leak when disconnecting inflator from Schrader valve?
I do two things - I overinflate the tyre by about one stroke of the track pump, and if the pump has a lever I flick it off pretty smartly.    However my pump at home is a thread-on track pump and doesn't have any issues with pressure loss when slowly unscrewing the hose from the valve.
Also, try another pump.  Borrow one from another cyclist, or check the LBS for an air line.  Some bike fixup workstations have a pump built in too.  Last resort is to try a car air pump at the local service station, though they can be quite brutal.
If you use a track pump, leave the handle in the up position while disconnecting the hose head from the valve, and you'll see the handle fall under its own weight.  I bet the HISSSS of air moving stops at the same time the handle bottoms out, suggesting that the released air is coming from the pump not the tyre.
Lastly for self-reassurance, check your tyre pressure by hand before removing the pump hose, and again immediately after.  You'll have a good-enough muscle memory for the pressure needed by your thumb to dent the tyre's tread.  I expect it to have exactly the same perception of resistance to your thumb before and after hose disconnect.

Minor clarification - the pin does NOT need to be pressed down in a schrader valve to add air.  If the air in the hose is high-enough pressure to overcome the internal tyre pressure plus the spring pressure, then air will go into the tube (barring no leaks or sticky valves)