Petcock

If you can find a petrol resistant sealer then it should work, i only fit fibre washers and screw them all the way in and then give the lock nut a good turn and never had a problem

 

Why not try threadseal you never know. You could use wellseal i think , maybe even rtv gasket but as dark says I would try nipping the locknut a little first, or as it is a rubber washer maybe very slightly undoing it , you may have distorted it out of shape.

 

You need viton seals.
I have a T110, and since ethanol was introduced I've had leaks.
The cure for me was on eBay LCBspares item no 322262858299 (Lancashire Classic Bikes)
Problemo solvo!
Taff

 

You need a Doughty seal. They come in all sizes metric and imperial. The screw thread on the pet cock should be British Standard Pipe Taper (BSPT) so fitting either a Doughty or fibre washer plus giving it a good nip should seal the leak.