The oring that seals the top lid of your Proline cartridge filter may need to be replaced. Never lubricate this oring with petroleum-based products (like vaseline). Teflon or silicone based lubricants will make the oring last longer. We have the orings and several other parts for the Proline filters at http://www.waterwayparts.com Part # 805-0460
Just to make things a little more complicated, there were several different versions of this filter. You may want to email a photo of your exact filter to ensure you get the right parts. sales@waterwayparts.com