Daren:

Thanks for giving us the benefit of your Merc tech experience. I was hoping you would see this thread and chime in.

Last year when I rebuilt the lift pump on my V-6 EFI I wondered why the check valves had changed in design, from what was in the pump to what was in the kit, and your explanation about the rubber material makes perfect sense now with the comment about ethanol.

Thanks for the clarification and I'm glad the OP solved his problem.