I always use a liquid pipe/hydraulic sealant smeared around the flange and around the thread. I never have any leaks, plus the sealing washer is important too, the ones the VOC spares sell for the oil pipe banjo's work very well as they are soft. I don't like the red fiber washers as these are too hard, except on the stock carb banjo fittings, and sometimes the soft sealing washers help out here as well. Another washer that works very well on the lever taps with the loose adjusting nut are the copper ones with the rubber insert, a bit like a Dowty seal, as they crush and this forces the rubber insert hard into the thread which seals it. Petrol taps are a real pain, especially when they leak internally..........This is a common cause for the original side bowl carb's to continually drip fuel even after the bike is sitting for some time. Disconnect the pipe fitting and leave a small container left under the tap overnight to see if it is leaking, you will soon know, this applies to the original push/pull taps as well which are notorious for leaking. Not good on a Black engine for sure..............