Stock sucks, haven't seen a way to repair them once they begin leaking.
The downside to aftermarket is the same as the stock...
Stock is 50% chance the lever is not upside down. Get the right aftermarket that fits and use gas resistant gaskets to adjust threads\pitch\rotation.
there's one with a twist valve at the bottom and pipe threads that never leaks
if red fiber washer is failing, I use a 1/2inch drill bit to chamfer the gas tank fitting just a bit to capture an o-ring that fits tight on petcock threads
In reply to those above, yes I installed the little plastic washer, I can't tell if it's the threads or the valve portion itself. I bought some permatex thread sealant I'm going to try if that doesn't work and I will have to try another valve.