Abstract: A balanced system for transferring a volatile liquid fuel into a tank therefor while avoiding passage of fuel vapors into the atmosphere. Vacuum assist means is provided in the system to compensate for lack of integrity of the fit between a removable fuel dispensing nozzle and the tank filler tube. The system includes a vapor pump to carry vapor from the tank being filled, and a valved bypass means to conduct liquid away without impairing vapor passage. The system also includes a check valve in the vapor passage to prevent escape of vapors during periods of non-use. The system further includes a trim valve connecting the vapor pump outlet to the vapor pump inlet so that the desired vapor/liquid exchange ratio can be attained during calibration.