Benjamin Franklin 317 .177 Valve Help needed.

Good Morning!

I just picked up a 317 .177 Having issues with the valve fully opening upon firing pellet. Seems there is still air in rifle and I can dry fire 2 to 3 times to get ALL air out.

Been watching Videos and looking for I believe A tool to remove the retainer nut on valve. Or your trick on how to remove it? Also where can I get a either a new one Valve or parts to rebuild it. The rifle seems not to have a serial # ?? Some videos say it may be very early 50's or late 40's?? Very nice rifle over all would be a shame not to get it working right. Any help would be very helpful. Thanks