So some way or another, it looks like the aluminum around the top of the plastic valve core has eroded and allows fuel to cross over at a high enough rate to cause the loss of reserve functionality. 

I can get the old screen off without damage, just need a new valve body if nothing else. Not in a rush, and looking for cheap. Currently in Pennsylvania. Send me a price. I'd prefer using paypal if possible.

EDIT: '91 GS500. No vacuum line on the back.

I have an old style and a new style petcock (i replaced mine with a CR250X)  Both need to be taken apart and fixed. You can have them both for $10 shipped.  I also live in Pa.