cartridge filter part question

[grpools]

Hello! I'm new to this forum (first post) -- here goes.....

I have a Clean and Clear Plus Cartridge filter and when I opened it up today to clean the cartridges I found the long cap (made of plastic) at the top of the pipe that runs down the middle of the manifold (or assembly?) disintegrated. I'm not sure what to replace it with or if it even needs replacement. My husband and I can't determine from the manual what purpose the cap served other than keeping stuff from going down the pipe (so it would be unfiltered and could clog the heater). If that's all it does, could we replace it with a cap that would fit the pipe (one from the plumbing supply store) or, does it serve a purpose beyond that and we need to buy an entire new manifold?

Any suggestions or assistance would be appreciated.