With a pump set up like that, the only place you can put a check valve is on the suction side. Your check valve way up there is not doing any good. I would put the check valve where the filter is, and put the filter after the pump. With the filter on the suction side, it will starve the pump for water and cause cavitation. Also the filter is just another place to get a suction leak. Any leak in the suction line will cause you to lose prime.