Rebuilt small block 400 had good oil pressure, after running for break in lost oil pressure. No knocks or rattles both sets of rockers pumping oil fine. dropped pan and changed oil pump no change. things I have checked hidden oil gallery plug is there,all plugs in oil gallery behind cam gear and the back of block plenty of oil pressure at oil filter. all new cam bearings installed at good machine shop.Motor has been run for a long period with no damage, just no pressure out of oil pressure sender hole. what in the world can it be?