Are you sure that the half-moon part of the gasket was seated properly when you remounted the covers, sounds like thatīs where the leak occurred?
Anyways, when I changed mine I used this sealant sparingly in the whole groove in the covers plus the spots in the pic below: http://www.permatex.com/products/Aut...sket_Maker.htm
It helps in holding the gasket when mounting the covers and seals everything!