I have a 1.5 HP pump for a sand filter. It looses pressure after filtering for awhile. If I backwash then go back to filtering, the pressure goes back up - for a while. We changed the sand (it was 5 yr old) and it seemed to help - for a while. Now, it loses pressure again. We dried everything off and watched for leaks, but could not find any. However, I do see air bubbles in the little tub when filtering. (Sorry, I do not know all the technical terms.)