I have a Pentair Clean and Clear 420 filter housing, circa 2007, that has developed a crack in the bottom. After reading related threads on the forum, I have decided not to attempt epoxy, and am replacing... but the question is, how much do I need to replace? The interior fittings appear to be in fine shape, as is the pressure gauge and exterior band. So, I am considering replacing just the bottom fiberglass shell ($269), or the top and bottom fiberglass shell ($610), or the entire unit, including 4 cartridges ($806). My local pool store says that I should have a matched set of the same production year for the top and bottom, because there are subtle differences in how the plastics expand and contract with pressure, and a new bottom may not meld well with and old top. Is there any truth in this, or is it just a tactic to get me to spend more? I'm tempted to replace only what is broken (the bottom fiberglass shell, @$269 plus installation)... because if I need to I can always add the top, right? I have 2 sets of replacement filters, so don't think I need the whole shebang. Would appreciate advice from the experts.