Thanks 911, that's some really good info there. I had no idea there was a check valve in the stem itself, I assumed it was in the lower housing somewhere.
I actually swapped filter lids/stems from one of my .952 engines stored in the garage hoping it would have better looking o-rings, but that didn't help much so I ended up buying new ones regardless. So not sure of the probability that the check valves in both original and current stems being both faulty, but I can always give a quick blast with air just to make sure. I'm heading home late tomorrow so might do a filter change while I have the lid off.

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD