That is the safetyrelief valve whichmeans it has gone bad or you need to turn back the high pressure setting about 5 t0 10 psi. When this happens again there should be a small pin in the middle of it that has a metal ring through it. When it goes off push on that pin and if the air coming out stops or shuts off then you need to turn back the psi if not the valve needs replaced.
below the pressure switch is a unloader valve which has a tendency to leak, undo that line and block it off, if this works you probabaly can leave it blocked off.
Question edited for correct spelling of maker 'Noen' to 'Moen'.
'Fix' as in it is leaking? Or tighten it down because it is wobbly, or something completely different?
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It is leaking due to a defective mechanical seal. It needs to be replaced. To replace it, is to take the pumpcase off by taking off the bolts that hold it. Then, take the impeller off to take the mechanical seal behind it. Put the replacement in, and reassemble the pump. Just get in touch with us by e-mail to get more assistance.
Move the lever or turn the knob. Check the stop tap under the sink. Make sure the faucet control knob or lever is tight, There is normally a removable cover that hides the screw..