Can the diaphragm of the pentax smart head be replaced?

Sounds like a faulty pressure switch. Contact the maker.

https://www.google.com/search?q=pentax+smart+head

Videos and earlier Fixya help

https://www.google.com/search?q=pentax+smart+head+pressure+switch

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new pump perhaps

To what camera does this statement (it is not a query) apply ?

This is not a camera

Check the pressure switch. Be sure you have enough water available. Check for a leak. What are you pumping. Is it pumping. I am not familiar with this pump so I am just listing some points to check. If it is a submersible well pump it can develop a lot of pressure.

There's not enough water running into the pump this is why its cutting out , hope this helps

yes, buy a better pump that is not smart

May be the problem is with the lens. Put a maximum aperture say f 22 then press the shutter, if the leaves of the diaphragm shuts and opens fast ten your lens is OK
.If there is any oily appearance between the leaves of the diaphragm then it should be cleaned.