It sounds like you have a faulty check valve in your power brake booster hose. If you don't know what this is; it is the large cylinder that sits behind your brake master cylinder (the thing you pour brake fluid in). There is a large vacuum hose that connects the booster to the intake manifold. The vacuum present inside the intake creates a low pressure inside the booster, which exerts extra pressure on the linkage to the master cylinder. Normally, the check valve only opens one way, allowing the engine vacuum to equalize the pressure inside the booster. If it is stuck open, then when you apply the brakes, the booster can change the absolute pressure inside the manifold, just when it is most sensitive to changes; when you have a closed throttle and you are braking. The vehicles computer sees that the throttle is closed, but the manifold pressure is off and adjusts the fuel mixture and timing accordingly - causing it to stall. Another possibility is that in addition to a faulty check valve, the booster may have a leak as well, but you would probably notice this as the brakes seem harder to press than usual. A decent mechanic can check the booster with a hand vacuum pump, and most auto parts stores sell these check valves for pocket change.