I replaced the sand on this filter, which I have done a couple of times without incident. When I fired the system back up the filter was leaking where the top cover meets the base. I made sure the screws were tight (the many, many screws to secure the top cover to the base of the filter. Today I took the cover off and thought maybe the TOP DIFFUSER was not tight to the INTERNAL DIFFUSER ELBOW, so I made sure to was tight. When I fired the system back up the filter began to leak again. Any suggestions?