The Harley Hummers use the same petcock and, like yours, they eventually all go bad. I have stripped several of them just to see what makes 'em tick. You can tear them apart by filing and messaging the pot-metal casting where it has been crimped to hold the outer face on either side. They have a circular rubber flap peened to the end of the shaft, when you turn the wing it "cams" the shaft outward which pulls the flap away from the inner concentric circle of the valve casting. What makes them go bad is either the flap gets deteriorted or swollen by modern fuel or it comes loose from where it has been peened to the shaft, or both. Putting one back together would require some way of evenly crimping the outer valve body back against the steel face which holds the stem. You can't use any heat or solder because it would melt the rubber flap. Bottom line is that they were not made to be repaired.
There are currently NO good repops available from any source, all the ones you see from the various resellers apparantly came from the same source (Tedd's?) and they are all junk, they either leak or don't pass gas (which in this case is a desirable thing).
I got one of my bikes back on the road by drilling up through the bottom (outfeed) end of the petcock into the central cavity of the valve. This will allow the valve to flow freely and use all the original fuel lines and connections. The downside is that you will not be able to shut the fuel off and you have no reserve, and you have to have a good needle and seat in the carb. But, it will get you down the road if you want to ride the bike. I can post a few pics of the whole mess if anyone is interested.
If anyone learns of new production petcocks which actually work PLEASE let us know!