It's normally caused by a split in the diaphragm in the servo. As you press the pedal it flexes the diaphragm and opens the split, allowing air in. This air is then sucked into the inlet manifold and weakens the mixture so much the engine dies. Best way to check is to disconnect the hose to the servo, block it off, start the engine and pump the brakes. If it doesn't stall then it's definitely the servo leaking.