Volvo MD1B oil pressure and laying up

[Pentland]

Hi all,
I got a lot of help here earlier in the year when I installed my Md1B. The engine is in good order and had been stored for many years. It starts very easily by hand and after advice from some of you re zero oil pressure I fitted a new O-ring on dip stick and squirted some oil down pressure gauge line. Hey presto, oil pressure immediately jumped to 50PSI and engine has run like a watch all season. If I left it for a week I had to wind her for maybe a minute by hand before oil pressure would register as it seems to drain back down into sump. Well, now I'm beat. She hasn't been run for maybe four weeks and no matter how much I turn her over I can't get any pressure. I've replaced O-ring and tried squirting oil down gauge line, but to no avail.
I can't find a decent diagram showing oil system and my engine seems different from illustrations in the manuals I have seen. There is no strainer like the MD1. There is socket headed plug beside dipstick, approx. 1/2" dia. but I have been assuming it was just to allow oil removal when changing oil. Maybe I'm wrong.
I'd be grateful for any advice. I'd rather try to fix it now than leave it to the new year.
On a related note, the reason I've been trying to run the old girl is to flush out the engine. I was intending to fill her with central heating corrosion inhibiter. Any comment?

[Lepke]

I haven't had one of these engines in a long time, but the socket head plug may be the oil strainer. It needs to be cleaned every 100 hours. But I don't think that's the problem. More likely, something isn't tight and you're sucking air or the oil is colder and thicker. Starting the engine will probably give you oil pressure. If you had 50psi, the pump is good, the strainer is clean enough, maybe the oil is colder and harder to pump???

[Pentland]

Thanks very much for your comment. Ididn't get to it today but tomorrow morning I will investigate strainer possibility. I don't think temperature is an issue as its been into double figures-- about 10-14degrees C and it never gets that low in the North of Scotland. I have tried starting the engine but after about 30 seconds have cut her out as still no pressure. I agree with you that she is drawing air but I'm struggling to think where. I would dearly like a decent set of drawing showing the oil galleries as at the moment I have no idea how it gets up from the sump.