Try this:
Set the front float level (primary circuit) slightly on the high side (not too much!!), and set the rear floats a little lower than ideal. It is unlikely that you will run the secondary bowls dry, unless you are doing sustained high rpm/speed driving, which unless it is track day, is unlikely.
What may (emphasize 'may') be happening is the fuel from the secondary (rear) bowl is sloshing into the choke tower and momentarily flooding the motor, enough to stall it with the throttle closed.
If you want to see if this is the case, set the rear float level to well below normal, so that the bowl is running almost dry, then do a hard brake and see what happens. If the exercise works, then slowly increase the float level until it starts choking a little on hard breaking and leave it there.
Remember not to run to the point of opening the secondaries while the float levels are set very low 'cause you will end up running very lean.
HTH...let us know if you come right.
Tim