Generally Automatics need more vacumm to substain repeated braking,...a manual transmission equipped car can get by with a min of 10 inches/vac, because the engine recovers vacumm when clutched, where as an auto's idle in gear drops the intial RPM by a few hundred or so RPM, causing vacumm loss, intial/ and repeated braking consumes even more vacumm, and an auto in gear is slower to recover,....but 14 inches should be enought?,....try hooking up a gauge between the booster and motor that you can read while driving the car, and see what real world demands are when braking,...before making any decisions/modifications


PS....you are pulling vacumm from the manifold, not the carb for the booster?, correct?...