My old Cortina V6 ran a blow through setup. The stock Solex 35 2-bbl throttle body had all the fittings in the standard postions. The solution to the C3 modulator valve was to get rid of the trans and replace it with a US 2.8 Mustang C4! Then you have similar problems with it!
My solution was to:-
1.run the 1956 Ford Prefect's vacum canistor (this allows you to build residual vacum under certain conditions).
2.The Impco LPG carb runs a balance line which allows the carb to communicate to the convertor. No fuel pump, no rising rate fuel pressure issues.
3.Limit boost to 6 pounds. (My IHI RB06 or whatever it was coded..it was too small, and was given to me by a work mate off his old Fiat 125P Turbo!). Downside was an engine with about 115 hp at 5200rpm only got to about 155 hp estimated...still lots of fun in a little 2450 pound car that would rev to over 6500 grand or more!
The brakes were always boosted correctly, but the modulator set-up was a real pest, and I never was able to make it shift as well as it should. The booster ran off the Prefect's vacum canistor, and the modulator off an old White Air meter reservoir used for concrete testing.
The Holley Performace book talks about boost referencing. If you can make the trans "see" the normal vaccum, you can do anything.
In most normal sitations, 6 pounds of boost is not enough to create a situation where there is no vacuum for brake boosters and auto trans modulation. Above 15 pounds, there sure is!