Remember that you are attempting to seal 4 joints / 8 surfaces. If you add an extra washer on each side you've increased this to 6 joints and 12 surfaces, which may also increase the problem by a similar factor! Far better to get the correct sizes of everything, IMHO.
I have been having an ongoing problem with fuel leakage on the carburettor banjo connection on my Austin 7. After many attempts at different solutions (copper, aluminium, fibre and Dowty washers), a car mechanic discovered a hairline crack in the inlet of the Zenith carburettor. I need to get this welded but at the same time, I'm strongly considering having a barb hose tail inserted so I can just clamp on a conventional fuel line and lose the wretched T connector altogether! Anyway, it might be worth double-checking where the leakage is coming from in case you have a similar issue.
