After back washing pool filter return pressure is fine then 10 minutes later it is real low .

Are you getting air bubbles going back to the pool returns? If so, it sounds like you have a suction leak. Check the pump lid "0" ring and make sure it is clean, not flat or cracked, and sitting in the correct spot on the pump body. When you see the pressure getting low open the air bleed valve on the filter and see if you are building up air pressure in the filter tank. This would for sure indicate a suction leak.