My local car spares shop sold me a piece of rubber tube with a slit in it. It has a rivet shaped stopper in one end . Simple but (possibly, fairly ) effective once I had made the slit a bit smaller. In truth, I have only used it to bleed mountain bike brakes so far, for which it was more of a hindrance really . Certainly cheap and was highly recommend though. 'Been selling 'em for 20 years mate !' The jam jar is a fail safe though isn't it.