Ok to clarify: Fork seal leak - a leak that occurs at the seal between the upper and lower tubes. A leak in this area can be caused by debris on the tube ( bugs, sand, etc.), damage to the tube from from rocks or by bottoming out the forks (by hitting a pot hole or like in my case railroad tracks).
Extension leak: Occurs at the connecting point of the extensions and upper tube. From comments on this forum that can be corrected by adding a larger O-ring or using teflon tape.