I recently bought a tire pressure gauge made by planet bike (see attached picture). It is supposed to be usable with presta valves. Every time I try to engage it on the valve stem, a bunch of air rushes out in the process and basically deflates my tire by 10-15 psi. FYI, all my bikes have sew-ups (700x20 and 700x22) with threaded presta stems.

I figure I must be doing something wrong, but what? Before this I was using a hand pump with a gauge, but now my building has an electric pump so I need a separate gauge to check pressure before I go out.