Some canisters drain and some do not. The 1955-1962 AC canister does not drain. It "drains" (outflows) high on the right side and fills through a metal tube that comes in the middle of the bottom and has holes in the tube high inside the canister. Thus, no drain-back.
I think that earlier AC by-pass filter canisters drained out the bottom (and you had to keep this in mind when drained and filled your oil (or checked your oil level).
Fram used to sell a unit called a "Sludge Siphon" (it looked like a bicycle pump - I will post a photo when I find it) to suck out the undrained oil in the canister and clean out the crud that had settled out in the canister (as non-detergent oil was apt to do).
