The invention relates to a dual fuel system for an internal combustion engine, a fuel injector for the dual fuel system, and a method for limiting leakage seal of the liquid fuel towards the gas combustion side. The dual fuel system includes liquid and gaseous fuel supplies and a fuel injector in fluid communication therewith. A gaseous fuel check in the fuel injector forms a leak path with a fuel injector body between gaseous and liquid fuel passages, and a leakage limiting seal is positioned about the gaseous fuel check and deformable into sealing contact with the gaseous fuel check responsive to an increase in a pressure drop from the liquid fuel passage to the gaseous fuel passage in the fuel injector.