What you need is this, never mind B18, B20B/E

1. a direct, unmetered or otherwise comprimised vac. line to the brake booster.

2. a filtered source of atmospheric pressure.

3. a means of metering vacuum & crankcase vapor for PCV.

So two clean intake ports, one with a metered orifice or PCV valve. One line with a filter on it somewhere.

You have both types filler caps, an integrated manifold one-port-too-short, and an HIF6 airbox.

I suggest you get a brass Y fitting, put it on the intake. Actually you'll have to rig one up out of 4 pieces probably, but I've done simmilar for gauges. Get the metered orifice from a B20 intake for the flame trap and the associated junk that goes in same... some kind of copper mesh I think. That one to the airbox. Use the B18 hose adapter on the Y for the brake booster. Use your B20 filler cap, route it to the intake too.

Or go the B18 route: 2x large orifice adapters on the intake, one to booster, one to PCV valve and an empty flame trap. B18 cap filtered. I don't like the PCV valves personally.

Old Kombi seems to have gone the B18 route, note the PCV valve on top of the flame trap. Makes all the difference. You can have it in line like he does if you trust that valve, but I would keep them seperate.

your local Car Quest or Checker or equivalent will have what I'm refering to with the Y. I can't recall the size, but they are very common standard threads. If it comes into question, I can measure tomorrow. Get some thread sealant too.
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'66 122s, '70 142s, '74 142e... Blue is beautiful.