Michel,
Are you sure the petrol is escaping around the washer? It could be escaping via the breather hole in the petrol cap. Most petrol caps have a double skin, with a breather hole in each. Now if the inner skin only has one hole, the petrol and vapour will be forced into the space between the skins, and then out of the vent in the cap - or, possibly between the skins. If the inner skin is modified to have a second hole, the vapour can escape via one hole while the petrol returns to the tank via the other hole. Whilst not providing a complete cure, this should reduce the problem.
To cure a similar problem with the screw on cap on a 2¾HP Douglas, I made a dished circular plate with a breather hole in the centre, which drops into the filler neck before fitting the actual cap. This provides a secondary baffle to prevent the petrol slopping directly against the breather hole in the filler neck.
Regards,
Eddie.