3 Residual oil on the sealing surfaces is probably the most common cause of leaks, assuming the cover is quite flat and a gasket or sealer is used. Clean and then re-clean the cover and axle housing surfaces thoroughly with a residue-free solvent such as brake cleaner. Did we mention to clean the surfaces really well? We like using Permatex the Right Stuff Gasket Maker alone to seal the cover to the axle. It is a thick elastomeric rubber material that can be applied to the cover circumference. A nice 1/4-inch bead works well. Apply it to the inside of the bolt hole areas.