Sediment bowl leaks. Is it soap rubbed on that stops the leak? It leaks out of the petcock seam and open close valve seam.
The valve is tapered brass. It should be polished smooth enough to conform to the cast iron bowl into which it is fitted. I have used timesaver for this job and it seems to work. I don't know how long it will last, but I have more timesaver if I need to do it again. It must be tight enough to seal against gas, but loose enough to turn. The soap is used as a lubricant because it is impervious to gasoline. The seal should be tight against the shaft and the iron. I have used the seal used by plummers on faucet shafts. Seems to hold up OK. Tighten it enough so that it is tight but you can still rotate the valve.
There is a lubricant for those valves also. It is expensive, and is available from Langs.
Soap has worked very well for me. If it is leaking around the valve stem, then I would disassemble the valve and thoroughly coat the stem with the soap "scum" from the bottom of the soap dish. I have had this work for years and years. I avoid ethanol blended gasoline if possible, as the ethanol will absorb water which might affect the soap adversely
I talked to someone about the soap trick, works for a while then the soap washes out and the leak comes back. In the past it has been suggested using tooth paste to lap it in. If it leaking where it screws into the tank, Permatex makes a white thread lock that seems to hold up to gas (even the modern stuff). My shut off has not moved in a long time so I have a shut off right at the sediment bowl in the fuel line. I don't have floor mats so it easy to lift the floor board, reach under and shut off the fuel.
EZ turn fuel proof lube from aircraft spruce. Super stuff.
I have tried soap, plumbers lube, and several other things, none hold up for long. First of all that taper has to be lapped to a great fit. Timesaver or such may work but I never had any so I use regular white toothpaste. It works well. Followed that with your choice of brass polish (I used MAAS), then clean well. Then I used the EZ Turn lube on the valve and the packing with excellent results. Sneak up on the packing nut tightness as you rotate the valve stem. Packing nut only applies to the type that has that of course (26-27). Early type without packing nut makes good fitting taper even more vital.
I lapped a Model A gas tank valve using toothpaste with borax. Cleaned out the toothpaste and reassembled. It worked great.