Thanks JackEB, you have shed some light on this for me! Andre's photos and measurements prove to me that my valves are not correct (and perhaps more importantly that I'm not completely losing my mind!). The first image below is my valve in the closed position. The second image is from Andre's page. The third image is straight out of a H&S brochure. Notice the distance from the bottom of the knob to the top of the valve body; mine is much greater than either of the other images (which I believe to be in the closed position also). Measurements show that my knob is simply not screwing in far enough to actually allow the valve to close. Now I just need to figure out why and see if I can fix it! It needs to screw in another 2.5-3mm in order for the valve to seat.