Just helped a friend with a complete overhaul on a side mount distributor 8N including the oil pump. On first startup no oil pressure. loosened some lines put more oil in filter and sort of primed the pump so to speak. Got around 20 psi on 30 weight oil when cold. Then dropped to looks like around 5. Four days later, started the tractor again, no oil pressure. Took the cap off of the filter and the oil had drained out of the filter housing with a little left in the bottom but not high enough to be at the level of the line on the side leading back to the block. Put more oil in filter loosed the line that comes out of the side of the filter housing and the gauge line where they meet at the block and kind of bled it again. We got oil pressure again after this. My question is: is there a check valve in the bottom of the oil filter canister to keep the oil from draining back into the crankcase? The reason I ask is that my friend poured oil in the filter canister and had forgotten to hook up the line on the bottom. He said oil only dripped out, not ran out quickly. I thought there may be a restriction in the bottom of the filter housing of some sort that we may need to check. We shouldn't have to prime the pump every time the tractor sits a few days.