You are refilling the shock pump with air from the shock itself.. It is a low volume high pressure system and that results in a big pressure drop when you re-attach the pump.
Boyle's law...Boyle's law - Wikipedia, the free encyclopedia
Sorry if I sound nerdy...
ok so basically if i put in 150 just let it be until next ride
If a fork or shock is working properly, there is no need to check it after every ride. More like 1/month.
The valve core will close before the pump un-threads. The only air escaping is from the pump hose.
The pressure gets lower when you reconnect because the air leaves your shock to fill the pump (as noted above).
my shock/fork pump works fine but when i unscrew the valve it always lets out air. whats the deal?
Is this a FOX shock with Auto Sag? There are two valves on those shocks. Be sure you're attaching the pump to the black valve (not the red). If you try to inflate the red one, the air will rush out when you remove the pump.
Salt Lake City 2012 Specialized Stumpjumper FSR Comp 29
2006 Specialized Allez Expert Double
I have a Topeak shock pump that you have to twist it onto the shock valve and then "engage" it to open the valve. You also disengage it before you remove it. It doesn't leak any air from the shock canister this way.
Works better than some others I've used.