Releaser control Valves

[Killington Maple]

Wondering if anyone beside me has had problems with the control valves blowing off the Lapierre double releaser. It seems that at the end of the day when temps drop below freezing and when the valves get a little internal frost on them, even though the sap is still running, thay will blow off the releaser resulting in a total loss of vacuum. This makes me reluctant to leave my vac pump running as it needs vacuum in order for flood oil to be drawn up into the pump.
Thanks for any input.