Leaking Marushin gas shotgun

[Ifritus]

Hi there folks, just wondering if anyone has ever worked on a marushin gas shotgun? This is the one where the NBA go into the tube at the front via three rotating brass barrels of 50bbs each, and the gas fill valve is located where the shell would normally go?

 

The problem is I've acquired this in trade and it's unfortunately sat unused for 2 years so it's obviously leaking... BUT I have no idea how to take it to bits to get to the gas chamber. (Obviously the smart answer is give it to a tech,but I like to learn so any assistance would be appreciated)