first, you have to remove your air filter first and check if the engine still stops upon brake application. If the engine dosent stop. then you have a problem in your air filter.
2nd, if the engine stop, check your linkage setting. There should be a minimum of 0.5mm~1mm opening on carburator during idle and even applying your brake. If there is no opening in your carburetor, then you got the problem adjust either your EPA ( end point adjustment on your radio), or adjust your linkages.
As basic rule, a minimum opening should be observed during idle & braking application.
cheers. . .
Xray Nt1 l Xray T3'12 l F104PRO