its the hcs engine thats in them (the tap monsters) I had this problem it turned out to ba faulty idle speed controller and it sorted my other sugestion is that there is indeed an air leak with it,

the only reason the brakes go hard is because the car has stalled and there is no pressure in the brake servo as the brake servo only works when the car is running, i wouldnt worry bout them tho cos if you press the pedal hard enuff they still work just a bit harder to press.