But i found this:
Stalling when you apply brakes can be the throttle linkage not adjusted properly or the clutch is locked up.
Your throttle linkage could be open a little at trigger neutral giving you a false normal idle speed and when you brake it closes the throttle all the way. Just adjust your throttle trim so it closes all the way at trigger neutral. Many people have this problem and most of the time it is a bad linkage adjustment. Your idle gap should be .5-1mm on that engine with carb fully closed.
Or your clutch bearings could be bad, and that's what is causing the wheels to spin at idle, and when you break the wheels stop spinning...then the engine dies. With a good clutch and linkage the wheels shouldn't spin at trigger neutral.