Dave Even if the sap lines freeze up on mine they will still put-20/-25 inches at the pumps. Do you run a recirculating line? if not that could be your prob at the one pump. Your getting air locked. Even the pump running at -10 I think is either air locked or getting to much air/gas from leaks. This year I actually had a leak I just couldn’t find until I was right at ear level with the tap. I do most my lines from a short ladder to gain height. Turned out it was the tree itself having a hollow centre. The recirculating line from the holding tank on a needle valve is the key to controlling the vacuum.