Several possibilities:

-Water Pump
-Poppet Valve
-Thermostats
-Powerhead or Adapter Plate Gaskets
-Water Tube or Seals

For the BEST RESULTS in troubleshooting.... always start AT THE SOURCE. Replace water pump components as needed... and then see where you are AFTER you've done that.

Think of it this way: you go out to wash your car. You grab the sprayer, and pull the handle.... nothing happens. What do you do? Hopefully, you go back to the water spiquot to see if it's turned "on"... and check the hose for kinks. Wouldn't do much good to take the sprayer apart FIRST, right?