I have a 1948 337 ci flathead in my F-7 truck. The original filter canister was leaking. It was replaced with another used filter canister. This one is leaking also. The top cover seal is not leaking. The leak is somewhere on the bottom. The leak could be comming from the lines, drain plug, or crack somewhere.
Is this a common problem on the old flathead filter cans???
your using the same lines/fittings, are you sure you don't have a cracked fitting or line? Those canisters are preety heavy. If you do find a problem in the can a good guy with a torch can braze or weld it.