There are 2 holes, one large and the small compensation hole. The seal lip should be just aft by .020" of the small compensation hole. The larger hole can still allow transfer of fluid when behind seal as it would push lip away from wall. The piston has to leave the compensation hole uncovered with brakes off, otherwise pressure does not bleed off. So it does sound like you have piston to far out the back and until you cover that smaller hole the pressure does not build up. Once seal covers small hole the piston are basically balanced and pressure starts to build.