You need a bit of mechanical advantage to pull the firing pin. Thus, the lever. I know it's not easy but I have been able to pull the pin out with my hand on every CPS gun I've worked on.
Take a look at this post:topic6265.html
Read through it, especially the paragraph below the trigger picture. See if that describes your problem.
I just had a thought: When you had the firing valve OPEN, you should be able to push the part with the rubber seal into the back half of the firing valve. When it is closed (but not pressurized) you should be able to pull the pin back at least 1/4", and most importantly you should be able to hear the valve opening (or not).