I don't know anything about the Convair ones, but I used to live in a place that had a Vulcan oil heater.
Over a long period of time a hard black gunge would build up in the firebox. The solution was to take it all apart, remove the firebox and chip out all the crud (take it outside - very messy job). I don't recall a filter, but it would make sense that there would be at least a mesh screen there somewhere. The oil was gravity fed. Our tank had a little valve on the outlet - it looked like a (brass?) concentric ring around the outlet connection, so wasn't obvious. Make sure nobody has messed with the valve.
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