the booster could be making the engine starve down by pulling all or most of the vacuum durning sudden braking. do you know what your manifold vac is? try this disconnect the vac line for the booster, cap the port on your manifold. try the same braking that was causing it to stall, please do this in a safe area. this will answer the question of, is the booster causing this. only way i know of to increas vac is to change the cam. some have suggested a vac canister, i don't know.