If your blaster shoots when you pump it you most likely have a problem with the firing valve closing all the way. This is common in older water guns. One simple solution to this is the rubber band repair. A rubber band is attached to the back of the firing pin to apply pressure and automatically push the firing pin forward, sealing off the firing valve, every time you release the trigger.
The following photos, provided by theSoaker, illustrate how to perform this repair. Hot glue was used in the following example to keep the rubber band in place, applied to both the end of the firing pin and where the rubber band wraps around the front of the blaster just below the nozzle. Additionally, aa type of waxed lacing used on airplanes to tie wire bundles together was applied perpendicular in order to keep the first rubber band snug against the blaster. You’ll have to message theSoaker on where to get that specialty part. Works great!