If the engine will tick over happily when stationary, but then dies if the brakes are pressed, this points at an air leak in the servo. The hose from engine to servo is probably ok as this would cause running problems all the time. Likely the servo diaphragm is split, or might just be the valving. You need a diagram of the inner workings, or a servo rebuild specialist, or a complete rebuild kit if you are competent to rebuild it yourself.