I found an old picture I took of my manual back when I had my Proto.
I thought, what the hell...
So I made a little leak/sound/performance troubleshooting diagram.
O-ring sizes will vary between fuse bolt systems. Consult your owners manual for proper sizes.
Also: For erratic cycling, first-shot-drop-off, leaks/hissing in the frame, and any consistency issues, check your solenoid, routing, LPR, and board settings (especially dwell/FSDO compensation if applicable).
MOST importantly, when troubleshooting ANY marker issue, return all board settings to STOCK. This will eliminate a lot of variables you'd run into otherwise while troubleshooting.
I run into a lot of players that are afraid to reset their board to stock, because they will "lose their 1337 custom tuned setup!!11"
Ladies and gentlemen, you can always write down your magic numbers before clearing them...
It's nothing big, and most people should already know this, but enjoy anyhow.
EDIT [7 March 2010]:
If you are replacing orings repeatedly, and are still having leaks, check the surface of the bolt/sleeve/seat. If the metal is scratched or gouged at all, the best oring in the world won't do a very good job of sealing. Yet another reason to use EXTREME caution if you have to use a pick to extract an interior oring. Brass picks are recommended, as the much harder dental picks can easily scratch/gouge the aluminum/delrin/nylatron/etc surfaces found in many spool systems.
EDIT [April 2011 (Yikes I am in my 20s and getting old!)]:
I've been informed that this diagram applies to pretty much the entire DM line, including the newer ones. Same for the Protos I believe. Obviously, use common sense: if you take out your bolt/spool assembly and it looks nothing like this, then the diagram doesn't apply.
I'd love to be more specific, but... I haven't had much time to actually play, much less tech!