I have found a solution for this...
In clickabledata.lua change this :
elements["pnt_344"] = default_button2(_("Fuel Dump Switch, ON/OFF"), devices.FUEL_INTERFACE, fuel_commands.DumpSw, 344, anim_speed_default)
to this :
elements["pnt_344"] = default_2_position_tumb(_("Fuel Dump Switch, ON/OFF"), devices.FUEL_INTERFACE, fuel_commands.DumpSw, 344)
And for those who have a winwing Throttle or a two on position switch you can add these two lines in defalut.lua (for joystick)
{ down = fuel_commands.DumpSw, cockpit_device_id = devices.FUEL_INTERFACE, value_down = 1.0, name = _('Fuel Dump Switch - ON'), category = {_('Left Console'), _('Fuel Control Panel')}},
{ down = fuel_commands.DumpSw, cockpit_device_id = devices.FUEL_INTERFACE, value_down = 0.0, name = _('Fuel Dump Switch - OFF'), category = {_('Left Console'), _('Fuel Control Panel')}},