Dr Nerf

None of the tanks on the official Supersoaker® replacement parts order form fit. However, I would measure the female end (where the tank screws in); from this photo it looks like it may fit a 1/2" male PVC adapter. If you have a 1/2" male PVC adapter, try it out in it. If it fits, find a way to build a tank or hook up a bottle to use as one.

I originally tried to use Dr. Nerf's valve design, but I've found it almost impossible to get the seals airtight.

I encountered similar problems at first but solved this problem by remeasuring and getting the right size O-rings. Originally, I bought and used the wrong size O-rings. I would also recommend using more O-rings than I used. Each set in mine had 2; however each set in yours may have 3 if the center divider is removed. I also replaced the O-rings in mine at least on other time since I built it because the system requires maintenance. Lubricated and with the right size O-rings, the system is a great semi-auto valve. Admittedly, I think the seals would perform better if they were not O-rings, but that was what I had to work with at the time.

Threaded check valves can be easily installed using the correct male adapter and teflon tape. Non-threaded check valves can be installed in exactly the same way that PVC pipe fittings are: using PVC cement and primer. When installing a check valve, make sure that the arrow points in the direction which the air should stay. Real check valves are far superior to schrader valves; I don't see how they can fail. If you really want to use a schrader valve, add in a redundant check valve to make sure.

It might be wise to coat all of the joints in the tank area and near the barrel and ball valve with some of the purple PVC primer, and then seal it up with a second coat once dry.

Right now the primer would do nothing. The primer is supposed to go on first before the cement, so putting it on now would do nothing. It is probably leaking because it doesn't appear as if primer was used before (or possibly because not enough cement was used). I don't know how many times I have to say this: primer is needed. It cleans the pipe and also (as a plumber once told me) removes a special "glazing" that was put on to protect it during shipping. If you don't put on primer, you are literally bonding the pipe to the "glazing", dirt and grime, and not the pipe.

Flaming Hilt basically told you what you need to do. Fill it up and put it in a tub (with water of course). The leaks would bubble to the surface. Mark where they are. If it is too large, just get a water bottle. Spray water on areas that could leak while it is filled with air. If water bubbles to the surface, mark it as having a leak there.

Next, you should try to fix the leaks. The only guaranteed way to fix this is through fillet welding. Putting epoxy or PVC cement on the effected areas may not work well. I made a post over at Super Soaker Central about fillet welding a while ago. Basically, it is literally melting the pipe with a torch to close all leaks. This scan from a plumbing book describes fillet welding throughly.

Edit: If you ever get the chance to repair it, be careful. I would never use unrated sewer pipe as a pressure chamber; it's just like asking for trouble.

I know I have said similar things to other people (mostly about PVC primer though), but it appears (based on the pictures) that you did not solvent weld it together or at least not use PVC primer. If it is not cemented, it will never hold pressure. And if PVC primer is not used, it will eventually leak.

I took of the legs but still the thing just wont get to any pressure.

Again, did you PVC cement it? If it was cemented, the legs couldn't be removed. It will just leak if it is not cemented.

If you did PVC cement it but did it cleanly, did you leave the valve open by accident while filling it with air? It could also be a bad input valve. The only way to fix that is to use a check valve and put the other input valve after it as CaptainSlug recommended.