This is irrelevant to Max-D blasters however, because the firing valve is a pull valve, not a ball valve.
You've got a common poblem for a rare blaster. The problem is indeed that the valve is getting stuck open because the spring inside has worn out.
Of course, you do have to open up the blaster but the screws are stripped and no matter what you try you cannot get them open. Generally, the last resort is to take a drill and bust up the head of the screw. If you choose to do this, pick the smallest drill bit possible and be careful not to accidently drill into the plastic. (You will know if you have if the drill suddenly moves very easily; thats when you need to step back and adjust your handling of the drill. A drill press may not be a bad idea if you happen to have access to one but the working surface may be awkward.) There may be other solutions out there for stripped screws but the only means most people have is to drill right through them. Don't worry too much though; if all the holes for the screw stay intact, chances are that you can remove what's left of the screw from the hole (by using pliers to unscrew it) and replace it with a new screw. This is only needed if the screw is occupying an important position, such as a nozzle selector. (Which is a VERY bad place to have a screw fail.)
Aside from screws, you have pump and nozzle caps to deal with. First, check if these items can be unthreaded by turning them counter-clockwise (as you would with a screw). If not, you should be able to use pliers or two flathead screwdrivers to pry them off. Be sure not to miss any screws that may be covered by the nozzle cap, and be careful not to damage anything. Of course, be sure no screws are on the nozzle itself. (Which I believe none are for the CPS 1500; I never handled one before.)
Now that the water gun's open, you have two choices. The target is the firing valve on the front of the blaster close to the nozzle. Take as many as the best rubber bands you can find (two to three are good) and wrap them around the back of the pin. This pin, as you'll notice, is connected to the trigger and gets pulled everytime you pull the trigger, which opens the valve inside. (Garden hose sprayers use a similar valve.) It should be possible to wrap the rubber bands around the front of the internals so that it has a place to hold, while part of the rubber band reaches around to the pin.
The second solution is less common but sometimes, in rare cases, rubber bands don't work so well. This method involves putting a small compression spring (found at the hardware store or from compression spring packs) behind the pin, but you'll need to improvise and find a place for it to hold. Usually, when using this method is a good idea, the PC case will provide a good starting point for the spring to rest. However, it will more likely than not be too short and will require a small flat piece (such as a small metal or plastic piece that's flat) to help the spring stay on.
This article provides a good overview of the rubber band method along with some pictures.
Good luck with the repair.
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