Do any of you guys know the size for the o-ring that sits between the black cap and the housing for a 4"x40" RO setup, looks like the one pictured below? The housing I have has been leaking from the endcap -- I think whomever replaced the o-ring before used one a bit too thin, and it is able to move a bit in the groove. The local hardware store has one labeled OR-340 which looks to be the same size as the one I think is too thin .. the diameter looks good, but needs to be thicker, and I don't know what the '340' actually stands for..
