HT75 Polesaw Oil Cap Question

[Rein]

Greetings! I'm working on an arborist friend's HT75 and the oil tank cap has a tiny cotter pin in a hole in the top. This obviously has not stopped it from leaking out of the cap. I'd really prefer not to purchase a replacement cap. Instead I'd like to figure out how to take this one apart, if possible, and install a new check valve, if that's how it was made originally. I'll add two pictures so you get exactly which cap this is. I suspect it can be disassembled but am hesitant to try until someone lets me know how because I don't want to destroy it. I know this is not a big thing, but I want to know how it works even if I have to buy a replacement later on. It has three holes spaced equidistant apart and I can sort of start to pry up (with a small pick tool) what I take to be a gasket around the outer edge, but it's pretty stiff - as though it's plastic, not rubber. Probing inside the opening the cotter pin was in I can detect a ledge of some sort, so I do think there was probably a check valve of some kind in there originally and ethanol or old age wore it down (like me) to a shadow of it's former self. Of course I could be all wrong and this thing was built this way and Stihl didn't care if it leaked a little, but I doubt it. ;-) I could, of course, plug up the hole completely, but that would probably slow or stop the flow of bar oil - something I'd be stoopid to even think about! I appreciate any and all thoughts about this and maybe, if I'm really lucky, photos of another one disassembled so I can see how it goes together. Oh, and I'm guessing the pin in the center which holds the chain in place just pulls straight out. Maybe that has something to do with the assembly? Just don't want to bugger it up wrenching on it with no information.