Well, I disassembled the top valve .. everything seems to be in working order, the valve gasket is in good shape. I've been playing with forcing water through it and I am wondering if it's not just building up enough pressure to blast past that gasket and go right out the return, but I can't recreate it so I guess there's not much more I can do on that front. I did tighten the lid back down onto the valve body as well as I could to ensure a good seal.
Here is the schematic for the filter I use .. it's dark outside so I'm not going to mess with it until the morning. They refer to the portion in the bottom as the "underdrain", which I would imagine houses what you are referring to, although the diagram is not clear.
Oh .. as far as backwashing goes .. that is one of my concerns. I'm getting around 15 PSI under normal filter operation. This does not seem to be changing at all unless the SKIMMER sock gets significantly clogged. The filter itself does not appear to be building up pressure. I backwash for a couple minutes and nothing really changes. I don't see much dirt coming out the waste side either. For as dirty as my pool has been, there should be all sorts of crud.