These valves can be tricky sometimes; the position it sits in while mounted to the frame makes a difference. If the fixing nut is adjusted too far from the frame, the valve activating lever can't reach the pin to open it. But if it's too close to the frame, it constantly pushes the pin in and the valve will stay open just a little bit. Difficulty level: high.
You may also want to purchase the steam wand kit.
Espresso Machine Steam/Water Valve Rebuild Kit Includes;
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.