Have a problem with low dp across barrel, dp switch wont make to start chiller. pump pressures 75lbs discharge 13lbs suction. only 4lbs dp at barrel, 5 story building. aaon chiller. found gycol pump not operational. pumped glycol in system with aux. pump suction side of pump went from 2lbs to 13lbs. still no change in dp across barrel. any ideas ?