I have opened an issue about it in the Bug Tracker. If you agree with it please remember to vote thanks!

LINK: https://www.tfbt.nuvturais.de/issues/1122


In the Bf 110 ( so far tested the C-4B and the C-4/N variants but I presume it applies to every Bf 110 variant available ) the Fuel Hand Pump is not working both if you assign a key to it or if you try to click with the mouse on the relative cockpit lever.