Once the cannon is assembled, you are ready to pressurize the air chamber. Start by closing the ball valve. Hook up your air pump to the air valve.
Slowly fill the tank. You want to do this gradually the first time because you need to carefully check for leaks. Start by pressurizing the air tank to 10 psi. Then carefully listen for leaks. Carefully open the ball valve to release the pressure.
Fill the tank again. This time go up to 20 psi. Again, check all the joints. If everything looks good you can take it up to 30 psi. Check all the joints again. Then take it up to 40 psi.
Let the tank sit with 40 psi for a few minutes to see if it loses pressure over time. If everything still looks good, you can be pretty confident that you made good seals everywhere.