Mike,
Some years ago a friend asked me to look at her Plymouth Duster because every time she turned left (or right–I can’t remember any more) it would stall.  It never stalled when she turned in the opposite direction.  I didn’t believe her until I test drove it myself.  I had a mutual friend with much more automotive experience, and I told him about it, and he didn’t believe it either until he drove it.  It turned out to be the carb float, which hinged on one side and was cutting off the fuel when centrifugal force took it in the wrong direction.  The solution was to rebuild the carb.  So there’s one possibility, if that’s consistent with your carb arrangement. (It’s been so long since I’ve worked on an air-cooled VW that I don’t know whether the symptom could be consistent with the VW setup, but I’m sure the VW gurus will weigh in on this point.)  Another possibility would be an ignition wire or other ignition component that might be shorting to ground when you brake.

I believe that the float would have to be WAY off to experience this. More likely, assuming it is not an intermittent electrical prob is that the carb mixture is too lean, and when you take your foot off the accelerator pedal to apply the brake, the throttle butterfly goes shut and starves the engine. Back off on the mixture screw a bit (see Rob and Dave’s VW resource for guidance). Increasing idle speed may also help. I think that VW recommends 850 rpm (check the number – i’m old). I prefer an idle speed more like 1000-1050.

Mike,