Diagnosis
The pump is turning but not pressurizing. This usually means the water is bypassing the pump internals completely. The first place to check is the thermal relief valve, a safety feature on the pump that opens if the pump overheats.
How to Fix It
Locate the thermal relief valve. On the MSH3125 pump, it's typically a small, hex-headed brass fitting on the side or top of the pump head, often with a spring inside. If the pump has overheated (from running without water flow), this valve can stick open. Try tapping it gently with a wrench to see if it re-seats. If that doesn't work, you may need to remove it, clean any debris, and ensure the spring moves freely. Sometimes simply removing and reinstalling it resets it.
If That Didn't Work
If the relief valve is fine, the issue could be severely worn inlet or outlet valves inside the pump head. These are little check valves that can get pitted or have their seals worn out, letting water flow backwards. This requires a pump head rebuild kit to replace all the valve seals and springs.
Heads Up
A stuck thermal relief valve is a symptom, not just a cause. It means the pump got dangerously hot, likely from dry firing or a restricted water inlet. Always address the root cause (check inlet screen, never run without water) or you'll just burn out the new pump.