Gasoline will kill these pumps. As will oil. If gas or oil got into your methanol, no seal in the world will stop that.
With straight methanol thats not contaminated.. you will get years of service. With gasoline in the mix.. 2-3 weeks it will start to leak. The valve isnt the issue, its the diaphram. Its made out of a material called santoprene, santoprene is excellent with alcohol, poor with petroleum.
Sometimes it can be a simple screws need snugging down, if the screws are tight.. you more than likely need a new pump..
Sometimes its the suppliers fault cross contaminating, sometimes its the user using old gasoline cans. The liquid should be clear and leave no residue.
HTH