more than likly water pump seals gone, also take a look at pump shalft,make sure its not worn and damaged the seals.also make sure the gear box breather pipe is not blocked up, it will coures a pessure build up in the gearbox.the impeller pump is plastic and they do break up some times,remmber to put the new seals in the write way round, some use a double liped seal or single liped back to back