Open up the blaster, mine was a XP 105 (Personal favorite from the XP line),
cut where the two plastics meet. For this gun it was on a lip made from a white piece over lapping a gray piece. Cut to about middle, stop, rotate it, and cut again. You want to cut all the way around this but not through it, as there is the rod in the middle.
In this case, the spring was so poorly rusted touching it with a screwdriver broke a chunk off. It's why this blaster stopped firing.
I went to my local hardware store and bought a few stainless steel springs that were thin in size. You want one as close to the size of the outer diameter rubber grommet and O-Ring combo in the back, without going larger than them.
Cut the spring to the proper size (determined by a little longer than how far the gap between the back of the front seal and the gromet are with the housing fully closed. Just push the rod up holding the housing together, remove the top, and it all should stay. Measure that)
Lifting the cover to check size, my spring is already there for that picture.
I used a dremal and diagonal cutter to do so. (I don't have any pictures of this, but if you would like I could take one)
After this I bent the now cut spring around the rod at the top and bottom with some pliers, just to make sure it wasn't going anywhere. The top and bottom are bent smaller.
I then took the vice pliers out again and clamped this shut, making sure everything is still aligned, and used some goop around the edges to help seal it, flattening the goop with my finger. Shakey macro pics are my cameras specialty.
Wait a day. VERY IMPORTANT STEP. ( No picture of this step sorry)
After it dries blow down the front and back, making sure no air goes out either way, just to be sure the springs the right length.
Then wait a bit more, because you can never be too patient.
After that wait epoxy putty over everything, with the vice pliers still on.
Here you could throw some mesh-tape on here to make it max out at really high PSI, but that's for a different mod.
After about an hour of this take the pliers off and put a little more on the tops. Connecting everything to make sure it holds.
Then slap it in the shell, make any adjustments with a knife if needed, albiet unlikely. From here you could put a little more on if you want or missed any spots.
And you're done!
It now works again, and should be able to hold higher pressures while also being rust resistant.
After here I did some general maintenance things, feel free to ignore this next bit.
I lubed around the O-Rings on the PCs, and then gooped the outside of them, just in case something would leak. You can take goop off later if you wanted to swap the PCs or something anyway.
I also lubed the pump
To make this fire better I hot glued all over the spring at the back of the firing rod/pin that the green part pulls. This way the trigger can pull this all the way without having to worry, so more initial force. I'll get a picture of that next time I preform one. I really don't like springs behind trigger pulls.
I put some more resin on the trigger bar, and the green part the trigger connects to just in case.
I lubed around the nozzle to make it better sealed and easier to remove. I'm going to put some kind of rope on the nozzle so if I yank it off mid battle I don't have to worry about it falling and getting lost.
And that's my mod! Questions, comments, flames?
P.S. Don't use imgur.
I got mine working without opening the pressure chamber/valve directly but forcing the valve close from behind(where the stab for the trigger comes out).