Is my pressure gauge broken? I recently got an xtech pressure gauge that reads up to 160psi. My bike is a road bike that uses presta valves on the inner tubes.

Here's the gauge (the one at the bottom):
http://www.moruyabicycles.com.au/contents/en-uk/d377.html

When I pop it on the valve the reading only seems to go up to about 40 psi. I feel if I inflate my tires much more they might explode (they are pretty much rock solid and I was expecting a reading of something more like 100) - they seem to have the same level of inflation as I bought them.

So my question is, is there a special way in which I should be hooking this up? At the moment I'm just unscrewing the metal release thing on the presta valve and pushing the gauge on firmly. The gauge holds its reading even after you remove it, and I've tried resetting it by pressing it's release valve button but that doesn't seem to make any difference.

I'm guesssing it's busted but maybe I'm doing something silly.
I suggest borrowing a pump with a pressure gauge and pumping up your tyres with that. Then try your gauge once you know the tyre pressure. Bike shops and a lot of cyclists have a floor pump because they work better than the little portable pumps than most cyclists carry. And those have a gauge on them that's usually fairly accurate.

You say the pressure release button on the gauge "doesn't seem to make any difference". What do you mean - does the gauge drop back to zero? If not it's definitely busted.