Only a week or two ago the same discussion was on VolvoAdventures dot com. All B20, B18B, and later B18A engines on the 144 had a closed ventilation system. Only the early B18A vented to the atmosphere. Also, the direction of circulation in the B18 is different from the B20. The B20 gets air from the air box sucked into the oil trap (red tear drop box) where there is a little metal flame trap in the tipple of the oil trap, then sucked through the crank case through the oil filler cap (with oil trap) through the hose into the inlet manifold. Obviously it is the low pressure in the inlet manifold that causes the suction.

The B18 works the same as the above, but the hoses are swapped: from air box to filler cap with flame trap, and from oil trap (with control valve) to inlet manifold. Some B18s might be the same as the B20, as there was a transition period. The B20-method prevents (supposed to) oil contamination of the air filter.

Not the answer you wanted, but now you might be able to work out what you need to do. As a help, the webmaster of VolvoAdventures (Jim) has an Amazon with a B18 drilled to a B20, and he kept the original atmosphere venting system. I think the oil trap (tear drop) is different though, because in our discussion we had some misunderstandings. But from what I remember, the cap does not have a hose and the oil trap has a hose that just hangs down.

Hope this helps...