The flow through those old filters was very minimal at best, and when they plugged up, probably none. I have found water, and sometimes pinhole corrosion in the cannisters due to the corrosive stuff that ends up there. Running too cold, incomplete combustion all can contribute to moisture and acidic residue ending up in the filter cannister. It's a good little project to hot tank the cannister, inspect its integrity and the restriction orifice on the inlet if there is one. GMC's have a very tiny inlet orifice, which must have been intended to control the flow to prevent the filter flow from bypassing the engine lubrication functions.
