If your blaster is leaking heavily when you're shooting it and/or after you've shot it, but pumps and pressurized without problem, that means there's either a problem with the nozzle valve or there is a problem with the tubing after the nozzle valve leading to the nozzle opening, itself.
As in most cases with internal leaking problems, you'll need to verify where the leak is occurring before any real suggestions can be made on how to fix it. In this case, opening up the blaster seems like the next step if you want to really determine what's going on.
