I'm not sure that the front ones have a valve either on mine actually! I replaced the hose, and there was nothing in it....
Anyway, i did it today, i took off the single one, made the hole just a little bit bigger, stuck one from the front on, and its simply amazing...!! I recommend that if the rear washer on your car isn't too great, then swap the jet for a front one. Or probably better, a slightly more suitable one from a different car