Tools required:
17mm combination wrench
18mm combination wrench
2mm hex key (optional
Begin with the machine fully cooled and unplugged from the power source.
Remove the top panel from the machine.
Identify the steam valve:
Loosen the retaining nut using the two combination wrenches.
To avoid straining nearby connections the following trick may be helpful. Using one hand, pinch the wrenches together like so:
Slide the retaining nut off of the valve stem. If necessary, remove the knob.
Next, unthread the valve stem until it is free from the machine. If the knob has been removed it may be helpful to temporarily reattach this using the two 2mm setscrews.
The valve seat gasket is now visible. Inspect this area and within the valve body for debris that may interfere with seating.
Note - Adding a little Molykote to the two o-rings on the valve stem and the threads themselves can help smooth out the valve's actuation.