My IT manual says to drain the cooler and tank before removing the pump by putting a hose from the scv to the filler tube. OK my question is why will this drain the cooler, tank and pump? This cause the pump not to build pressure? Need a lesson please. I'm just wondering b/c I need to put a case drain on the 4650 for my schaben sprayer pump return. We are currently returning the pump through the scv and hydraulics are not working correctly to operate the sprayer. The local schaben dealer everybody runs the case drain for the return side of the pump on the sprayer. Not sure why we didn't get the message. So by returning the pump oil to the case drain will this cause the pump to cavitate?