Are you sure that it is the rear main seal??? It could also be the rear pan seal that if not installed carefully have a tendency to leak. Must make sure that the grove in the camp is very clan and dry and that the indentations in the cap/block are clean so that the end tabs of the rubber fit up tight in these. I have had the best luck putting this seal in dry and using just a tad of silicone in the coners where the pan gasket meets it.