A common question is "why does my rear shock always lose air?"
Frankly it is pretty uncommon for a rear shock to have air issues. However, the PSI to your pump gauge will drop for a couple of reasons on a functioning shock.
The largest PSI drop is from the negative air charging during the first few cycles of the suspension. This video goes over the basic concept of pressurizing the negative air chamber and by increasing the overall volume does decrease the main chambers PSI.
Check out more of BikeCo.com's Tips & Tricks such as the seemingly ever controversial Pre-Charging your Pump!
bikeco.com/precharging-a-susp...
or other Basic MTB Setup concepts:
bikeco.com/bikeco-tips-tricks...