I have a plug that goes in the pipe joining the cold sap float box to the flue pan, but there are 2 adjoining pipes with a partition between them. Each compartment has a drain valve, I plug that compartment, drain it, then remove the plug and that refills from the flue pan. Then I repeat for the other compartment.
The first compartment is where the sap enters from the pre-heater, the second one is where the float is, so it doesn't get affected by the entry of sap's turbulence, just controls the flow according to the flue pan sap level.