The tank to petcock o-ring differs between early manual units, the '78s, and the '79-on units. It's obvious just looking at them if you have them side by side. The 4-hole rubber disc is also a different size. Again, obvious if compared side by side. Rebuild kits can work but not in every single instance. Sometimes the petcock is worn or damaged internally and new rubber just won't help. If the petcock sat for a long time with fuel residue and gum in it, that can take it's toll, eating away at the inner sealing surfaces. They get all pitted and sometimes can't be rescued.
The other surface you need to inspect is the hole in the backside of the petcock that the vacuum powered plunger w/ o-ring fits into. It has a bevel at the top that the o-ring on the plunger wedges into and seals against. That may need some cleaning and polishing.