nope you dont need to let the air out to install a power adjuster (as in a hammer spring pre-load adjuster). Just pop it out of the stock, remove rear end cap, install bushing or whatever your adjuster screw is going to push on and reinstall. I used a 30 cent screw from Home Depot and a plastic washer I had laying around, cheap!
Now on most all pcp's I have had (and the Disco for certain) if "I" wanted to let all the air out I would fill untill the check valve in the rifle opened, shut off scuba tank (or stop pumping) and VERY slowly turn the pressure relief knob JUST untill it starts leaking air, just let it all leak out to bleed valve, safe and simple.
John
