Both a presta and schraeder valve should close almost instantly once the chuck is removed unless it gets hung up. The air that you hear escaping is from the pump and hose.

How are you checking for the pressure drop? If using the pump's gauge, air filling the hose is enough to drop pressure. If using a pencil gauge, it's pretty easy to bleed off air before getting a reading. Plus those pencil gauges aren't very accurate. As a matter of fact, pump gauges aren't that accurate. I had a pump that read 20 PSI low at 100 PSI. In other words, it said 80 PSI.

As long as 'slightly over inflate' doesn't exceed the tire or rim recommendations, I'd recommend just doing that.