Is there a pump / Schrader valve adapter that prevent leakage during disconnection?
Much of the drop you see will be due to the reconnection, not the disconnection. 

When you disconnect the pump you have 120psi in the tyre and the hose, gauge and a little bit of pump. This immediately drops to zero (as these are all gauge pressures). Most of the hissing you hear is this air leaving the system 

When you reconnect the 0&nbsp;psi in the hose etc. is averaged with the 120&nbsp;psi in the tyre, weighted by their volumes (a slight approximation but it will do).

You can test this as you have Schrader. You can get a little tyre pressure gauge from a car parts shop that has very little volume inside (do the experiment at at lower pressure if it doesn't go to 120psi). Pump up the tyre. Measure the pressure with the gauge a few times, nothing the pressure each time. Connect and disconnect the pump. Measure with the gauge again. Repeat until you're convinced.