sap flowingbackwards

[Bruce Stuart]

What might cause sap to flow backwards in a lateral or mainline

[Russell Lampron]

When the sap is still flowing but the mainline freezes the vacuum in the tree will pull the sap back up the lateral and drop. Any sudden change in the vacuum level like when a mechanical releaser dumps will cause it too for the same reason.

[Walling's Maple Syrup]

If you are using a vacuum pump, a leak somewhere in your tubing system will also make the sap move backwards.
Neil

[maple flats]

Gravity vacuum from 3/16 will do that too if you have a leak or if a line freezes farther from the tree but not right at the tree yet.

[Ontario Ian]

slope could be a problem