hitachi WT-PS250XS weak pressure

[OldBird]

Recently the city turned off water without notification. Like many Thai homes we rent out the front portion of our home to businesses that use water leading to our tank running dry and the hitachi WT-PS250XS that feeds water to the home overheated and shutting down. Once the water was back on and the tank full again, i opened the top plug and filled the pump with water. Closed the plug and then turned the power to the pump on.  The pump turned on and sounded like it always has since the day it was installed brand new, However, since the day of the overheat the pressure has not been the same. The pressure is much weaker then it was and even when the pump is operating the pressure is nothing like it used to be.  Anyone have idea's why this is? I initially thought some gunk from the bottom of the water tank could be clogging or gumming up something in the pump. So i disconnected everything and pretty much took the whole thing apart to inspect the tank and all the other tubs and parts for wear or corrosion, I found nothing. Reassembled the pump refilled the pump and i am right back to square one. I did notice that there is a spring under a cap with 3 bolts to the left of the top filler plug. Could it be that the spring is getting weak and allowing water to flow at lower pressures?