I have two vacuum taps - one for the vacuum advance and one for the newly installed PCV (problem was happening prior to the PCV being added). Under hard braking, the fuel can slosh or move only in the tank and the fuel bowls. That being said, could the fuel pump pickup not be getting fuel? However, the carb should burn the fuel in the bowls. If the fuel in the bowls "sloshes" away from the metering blocks, could that starve fuel creating a lean condition? My thought is the fuel would move away from the primary metering block, and in to the secondary metering block. So maybe a higher float level in the primary bowl would be better? I'm sure you have it, but here is the link to the manual
http://www.barrygrant.com/fromBarryG...n%20Manual.pdf