You could try an equivalent from another manufacturer, see if that helps.
Seems though that the HF147 is a rather standard filter, comparable to the Champion, and I do not see how prising out the rubber seal may help get a better return. It would seem logic that the oil would leak out without the seal, so if it doesn’t that would mean there’s almost no return pressure.
Following that, I am leaning towards TT’s opinion that the filter may not be the culprit here but that there’s something wrong on the oil pump return side
When you have emptied the sump, does it take a while for the oil coming out of the breather pipe or is it coming out immediately?
I would start by disconnecting the filter feed line (engine return) and see if there’s a good return flow. Just don’t let the engine run to long in that condition because there won’t be any oil going to the rocker box. Or If it’s not possible to disconnect the feed line, at least check the return in the oil tank. There should be a decent amount of oil gushing out of the return pipe.
