Try a saturn water key. It allows more water to drain out each time you use it. OR You could try one of those Olds type water keys with the long lever (like a trombone) so you can empty without taking tyour fingers right off the instrument. I have this problem with my Bach C trumpet. I also get water build up in the back of the bell which is usually a cornet problem. I am considering changing the water key. One very annoying feature is there is no third valve water key and no removable slide on the end of the slide like on older Bachs.