I was wondering if it is ok to run it instead of using the non-pressurized one. This is for the neck that is close to the engine and the air pump. The AST is using 0.9, so that's ok. Just wondering if I'll run into problems if I use a pressurized cap for the filler neck.

 

Or does anyone know where I can get filler cap for a decent price. My local Mazda dealer wants $33 for one. That's more than a Mazdaspeed pressured radiator cap!

 

No, it is not okay to run a pressurized cap on the filler neck (the one you describe as being close to the air pump).

The system is not meant to handle a pressure cap there; a pressure cap there would do no good.