The return fuel line will reduce the possibility of vapor lock as the fuel can circulate back to the tank that is being held back by the pressure regulator. Plus, pressure regulators tend to work best when they can bleed off excess fuel via a return line.
RobbMC indicates : Allows “vapor return” line to be used with RobbMc pump. Reduces the chance of vapor lock and improves hot starts.
On hot days, fuel vaporization can greatly reduce pump efficiency and cause erratic fuel pressure. Vaporization can best be reduced by keeping engine coolant temperature under 180 degrees F. Vaporization can also be reduced by using a vapor return line, running race/aviation gas, using a “cool can” to cool the fuel, rerouting the fuel away from the exhaust, wrapping the fuel lines with insulating material, and by reducing the restriction in the lines by using larger lines or fewer bends. ~If a vapor return line is connected to the regulator, the RobbMc VR fitting must be used. Using a different fitting will cause a severe loss of pressure unless the fitting has a .030" to .040" diameter restriction.
