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Well, I got curious so I did some research in my Maintenance Manual. I have a 1967 T337B model - turbo with aux. tanks, 128 gals. total.
My POH says basically what others have said here, BUT the maintenance manual gives a clear diagram of the system. And the description, in paragraph 11-6 (Turbos) of the maintenance manual, says:
"Fuel vapor return lines return vapor and unused fuel from the front engine-driven fuel pump into the front fuel line manifold, where the fuel is recirculated and the vapor is returned to the left fuel tanks."
Then is has a similar sentence, with "right" where that one says "left."
Now I don't know why it sort of separates the word "vapor" in that sentence (it doesn't do that in the description for the non-turbo A and B models - it simply says the fuel and vapor is returned to the main tanks and the manifold).
BUT if you look at the diagram for the turbo models (through B series), it CLEARLY shows that the return lines go to both the manifold AND main AND aux. TANKS. So that means what I've been told from the time I bought my aircraft may be incorrect: "Burn about an hour of fuel from the mains (if they are full) before switching to the aux. tanks because otherwise the excess will be vented overboard." It could be that advice came because of what the diagram shows for the non-turbo models (through B series), in which case it shows the return vapor line going ONLY to the main tanks and not the aux. tanks also; in that case I can see where that old piece of wisdom applies.
In my case if it returns to both mains and aux. on each side I can't see why it would make any difference whether you burned fuel from the aux. first or mains first (assuming both are full to begin with).
But I beg to differ from those who claim that they don't see how it could get back into the tanks. On their models that may be so; models differ over the years and their diagrams and models may be totally diff. from mine. But on the A and B turbo models it shows venting back to the tanks as well as the manifold.
There is a discrepancy between the POH and the service manuals. The POH says nothing about feeding back into the tanks but only the manifold.
None of this yet answers the question for the 1969 turbo, though. If the jcthomas' 1969 model is a "D" then the only diff. I can see in the diagrams for that year turbo model is a microswitch, evidently fow switching a single gage from main tank to aux. tanks.
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