could it possibly be that the gasket was not seated tideley on rebuild? if it was a bit mis-placed or tightened down a tad to hard could this have caused it to bulge out so to speak? will it sit back in its groove or is it all mis-shapen and distorted? if it is, get a new one, they are cheap enough...and dont go over board on tightening,maybe when fitting you could use a VERY SMALL tack of adhesive in each corner just a little spot to help retain it in positition, my guess is it may have slipped out of place as you tightened the filter housing together......
