Here's a basic guide to some of the parts of a water gun
. I recall that iSoaker has better illustrations with more details, but the site is down right now. There aren't that many videos about water guns unfortunately, compared against Nerf which has a lot of video tutorials.
Sounds like you have the leak under control.
As for the CPS 2500 which won't pressurize, you'll have to give more detail. What happens when you pump? Describe it step by step. Let's say that you start with the pump pushed all the way into the gun. As you pull the pump out, does it seem to be sucking water from the reservoir? If no water is being pulled in at all then the problem is likely a clog in the intake check valve. If water does enter the pump, what happens when the pump is pushed in? Does the water seem to go back into the reservoir? That would indicate that the intake check valve is clogged in an open position. There are other possibilities as well. This is just what comes to mind.
Likely you'll need to clean it somehow. Typically there's a mesh screen at the intake to block debris from entering the valve. Check that first. If debris got into the valve, I imagine using "pipe cleaners" normally used for small crafts could probably help remove it. Flushing the valve with a lot of water by pumping repeatedly might also help.