Hi SS, I have an old Pac Fab (PENTAIR) FNS Plus DE Pool Filter. The manifold does sit on top of the array of grids. However, I always remove the manifold and pull the grids out separately to clean. I have inspected the grids, manifold, manifold O-rings, and even the air bleeder (air relief tube?). None seem compromised.

I am, however, headed down your path. As I alluded to earlier, the only question is, will I replace individual field-replaceable units (e.g., grids for ~$200), the whole grid subassembly ~$400, or will I just swap the whole unit for a new DE or cartridge filter for ~$1,000. I am very curious about the new cartridge filters that are supposedly long lived and low maintenance.

Did you replace the grids because you actually found the tear(s)/leak(s), assumed them to be the most likely culprits, or wanted to eliminate them as a possibility?