You might want to pull the filter housing off and check it over for hairline cracks, not a very common thing to happen, but not impossible either, and would explain the ongoing issues, It should seal right up by hand tightening the filter, and just firmly snugging the banjo bolts and top bolt, the washers are made either of aluminum or brass, seem them both ways, and they crush very easily, I've had good luck reusing them unless theyve gotten distorted.

If you really want to make sure it's not cracked, you can get magnaflux in a 2 part rattle can kit, works really well.
-Chris