Here's my advice on rebuilding this petcock, for you to consider. Do not buy a rebuild kit. Go through the steps above regarding cleaning and etc. Unscrew the vacuum barb and replace it with a short screw with the threads coated with seals-all. Coat both petcock halves with seals-all and screw them tight without diaphragms or the wafer and let it sit overnight. Use an inline knob as your off position. "Run" will stay the same and reserve can be either reserve or prime.
In my experience, the problem with the Special petcock is getting a pintle o-ring that will seal. The one that comes with the only rebuild kit I know of did not seal for me. While you try various o-rings the diaphragm takes on the characteristics of a used gasket. I ended up using the kit, with my old o-ring, since the original problem was the diaphragm, and it worked fine for awhile. I have always failed to find a new o-ring that will seal, even when they look identical to the original. You can stretch out the spring to give it more power, and it may seal except when the tank is full, when it will drip.
I recently had a diaphragm hole and leak that I thought was a loose elbow. It turned into a steady drip and I fixed it for the time being using "seals-all". Gasoline here is just whatever. No Shell station on the corner. In fact the higher octane gas up here seems to have a higher percentage of ethanol. It raises the octane or octane equivalence.