You have a bit of a mixture there. You have a B20 so that system has to either work as designed or changed back to one of the B18 systems. As said look at the diagrams on the SWEM page or in your Haynes if you have one. The basic difference is that the flow through the systems are the opposite way round. Yes you can blow through the cap because basically it's just a plain tube. Only one rubber ring needed but you can tweak the 2 small tabs to set the sealing force if needed. On the B20 the flow is inwards from the inside of the air filter. Therefore clean air. If you take the cap off and put your hand over the hole you shouldn't feel the air wanting to come out. If it does your system is closed the wrong way. Do you have the small PCV valve in the hose just above the ear? If so is it fitted the right way round. Not sure why that hose is so long but not a problem. If the engine runs OK otherwise you probably have the correct nipple in the manifold at the other end of that hose.
This problem comes up frequently and it's always down to the wrong set up for the engine. Volvo, bless them, did try and keep the fumes in the engine with the B20 and that can only happen if you stick to their routing and valving.
The adaptor shown only works with the alloy back plate that you have on your rear carb. If used it needs to have longer studs fitted for bolting to the front carb.
I note another small connection on your inlet manifold. I assume that is closed off. It was probably a connection for a vacuum gage. at some point in the past.
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