I purchased a 56 TBird 312 Fordomatic transmission last fall that needed work. Loadamatic distributor vacuum advance leaked in both diaphragms, Holley 4000 carburetor secondaries not working and numerous vacuum leaks.  Completely rebuilt engine, carburetor, distributor and a NOS vacuum advance.  Runs excellent now, except the maximum advance I can get is 20 degrees.  Manual suggests it should be 26-28 degrees.  Any thoughts where the problem might be?  I've read several posts that suggest the red check valve ball in the distributor port on the carb is not required in automatics, but not sure if this is related or not.
Thanks, Don