I have a Francis Francis X5 and when I open the steam knob valve to froth milk, it spins infinitely and will not close. Why is this happening? Any help is greatly appreciated.

I've never seen or used an X5, but my first thought is that the valve that lets steam through has simply become disconnected from the knob, and got stuck in the open position. You'll probably have to take the case off, find the steam knob assembly, and look that way. You may be able to access it from the knob itself, though I'm not sure. It probably has a small hex screw holding the knob to the valve pin.

Good luck!

Maxwell Mooney is correct in his appreciation of the failure, it just takes to apply a little excessive force to the dial, forgetting the fact that you get full steam with just less than a fourth of a turn, to disconnect the knob from the valve.