I have this exact same problem, being in Canada as well... The only place I've found any decent sizes or parts for PVC or CPVC was at a store which specialized in piping.
One last bit: Before he solvent-welded an OPRV in, Zorn's Big Blast went through three sheets of cardboard held up to the gun, through another cardboard box on the ground, and buried the dart in the dirt. And it wasn't breaking two hundred.
Yeah, this has to do with a lot of things. The size of the tank, the pressure the tank can hold (that you can put into it), the size of the hole where the air escapes, the type of air-release system, etc.
TC, please elaborate your question. Either that or go out and buy both of these. Personally I'd say the Cobra is nicer on it's own for modding, but I'd get a Titan combo pack for the same price around here and that's worth more to me. Eventually I'm sure I'll have both of these guns as they both have their own unique features.
This is wonderfully creative, the first thing I thought of after this was to add a breech for a Nerf clip, I think you could easily set it up to work with the pump, or not...