Recently switched my water filter system to this MSR guardian purifier from a MSR miniworks . I have exchanged this pump 5 different times from REI. Every single one ends up leaking from the seal behind the plunger for the pump on every stroke.

At this point after a few of these pumps all having the same leak makes me think this is a fairly common problem.

Has anyone run into this before? How did you fix it?

Thanks!