If anyone was wondering what the pressure retaining valve actually looks like - here you go. I replaced one today on one of the silver stars I have. You can see it when you remove the oil filter. I had oil draining down from the frame/tank when the bike would sit for a while - evidenced by oil pooled in the breather hose that goes to the air filter housing.
It's no fun to replace, I needed an impact driver (not the pneumatic or electric kind) to loosen it (and I don't think it functions anymore regardless of its condition previously). I wouldn't replace it unless necessary. There is a procedure for bleeding the air out involving removing this valve and kicking the engine through until oil comes out of where the valve was, but my interpretation of the manuals I have seen is that the bleeding is only necessary if one disconnects the oil lines - not every time you change oil/filter. I did this procedure once (different bike)- I was able to remove the valve without damage using the same impact driver, but I think I got lucky that it still worked.
You can see there are four parts. I have pictured the orientation they were in. Note the pin has a detent on one end, the end that goes into the rubber bit.
I will report back as to whether this solves the problem of the oil draining down from the tank. I am optimistic that it will.
If you are planning on removing the valve I think it would be a good idea to have your replacement parts in hand. Certainly it is possible to remove it without any damage and put it back in and have it work perfectly - but might as well be prepared if things don't go well.
Have fun all!