Stan , please do not take any offense in my "holding-on" to this subject..

But , please...

I have setup some pictures on my "help-site" and please open a new window and find this link:
http://www.sitecenter.dk/classicvolvoshop/crankvent/view_all.nhtml

Picture 1:
Scanned from parts-list - this shows the correct fitting of crank-vent hoses.
From oil-filler cap (very positive air-flow), direct to the manifold,
From crank-house (very variating airflow), direct to airfilter.

The engine uses the vent-house on side to "breath" so both a little vacume and sometimes opposit - outlet-ventilation. This is also why the vent-house is fitted with an OIL-TRAP and not only a FLAME-TRAP, actually. The vent-house get's air from airfilter-inside also to assure it's "clean" air, just as the carbs.

The nipple that goes to the manifold for the vent-hose from oilfiller-cap, is with a very tiny hole, apr. 2 mm. - apr. 0.08 inch - Due to the fact, that constant airflow is in this hose, therefor little hole keeps "false-air" symptoms away.

Picture 2:
From a original owners manual, 1968, B20/B20B - just to let you see an original Volvo picture of the setup.

Picture 3:
This is from a Volvo Service Manual, the B20 - showing and in words there is a explaining of the airflow for the crankhouse.
As you can see - blue arrows are fresh air - red are "used" air.

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For the brakebooster, the nipple is quite different from the other, it is with an almost "free" passing of air, apr. 5-6 mm. - apr. 0.25 inchs.
On a fine working setup - there would be no "movement" of vacum - no false air in system, only constant vacum on brakebooster.
Normally when braking, engine is deaccelerating - and thereof only little vacume - and you probably want all you can get!

To give you an idea of how much airflow there actually is in an engine (other than air-into-carbs, naturally) try this fun thing:
Hose the oilfillercap and venthouse together, run engine and look for the oilstick! - what happens? - yep, the oilfiller-stick is blown upwards and you should be able to detect oil trying to get out there too..

I KNOW that the 1967 model 123 GT was fitted with these T-nipples and "one-way-valves" but I would seperate the systems!

In your picture, your engine is fitted with an aluminium-intake manifold - this is originally from the early models, and I would check the nipple you are getting vacume from - can grease-up and so - clean and inspect and is it a small or big hole?

Anyway:

- either you could have false-air symptoms
- or uniffecient brakeboosting.
with your current setup, no matter the "one-way-valves".

Finally:
Clean the little air-filter for the vent on the brakebooster (right in righthand-corner of your picture) - and if brakefluid is in the booster - it's surely dead!

Sorry for the long post - just felt that a better explaination was right and I hope you get my english..

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/Soren