Engine has 15 passes on since it was totally rebuilt.Has new billet main seal deal from Mancinis.Now trust me,,, done everything i should do TWICE and after a run or a good warm up it leaks...Someone said it could be crankcase pressure????? I have seen these new rear main bearings that have a groove away from the seal that are supposed to take pressure away from the rear seal....Any ideas would be a help...thanks in advance

Did you offset the seal in the retainer so that the parting line is not at the retainer parting line. Had this work when nothing else did.

There have been many posts on here about this problem---many engine builders noticed about the time Fel Pro started putting the 'pipe cleaners" in with the rear seal that we all started having trouble with leaks--Not because of the side seals--most folks were smart enough to not use those stupid things but---the seal diminsions are different and the seal is different--Fel Pro says nope..same seal but it is not.
There is no substitute for lining the seal holder up on the block before the crank is in and scribing it so when you put in back the seal grooves in the holder and block line up--and now that the paper side seals are no longer supplied ( they actually helped center the thing) it is a must to check this.
It goes without saying that there are many other things that make it leak--crazy high oil pressure, crankcase pressure, a nick on the crank, lack of or wrong lube on the seal, oh boy....it can drive you crazy--The anti leak rear main will help but Mopar is the only place I know to get it and sizes get to be a problem there--Good luck and just know that MANY other builders are fighting this--makes a guy want to go back to rope style seals but then WHERE do you get them anymore? Good Luck!

You sure it's main seal leaking? , could be gallery plugs , cam plug or hopefully valve cover gaskets.......................some will install seal into retainer with silicone sealant , when the retainer gets bolted up the sealant gets over the lip & sticks it to the seal , bit like a sticky envelope.

Some billet main seal housings fit too close to the rear main bearing and don't let the oil from the rear bearing drain to the sump. The only escape route for the oil leaving the rear bearing under pressure is through the rear seal. Not sure what the Mancini housing looks like but have a good look at a stock main seal housing and you'll see what I mean.

Thanks for the replies.Took the engine out over the weekend.Thanks for the help Grumpy Ordered one of these from Mancinis (see Picture) Might as well try everything while its apart...And thanks again