Look down in the rear piston "thimble" in the back of the MC & there must be a shallow but wide circular groove about halfway down for the neoprene piece to lock into as opposed to it being a smooth taper like a sewing thimble, otherwise it just pulls back out like it is doing. manual brake MC's must have that recess. power brake MC's can function with or with out the recess cuz they are setup different & as you know you want the right piston bore diameter for your app. EDIT Rick Ehrenberg iirc used silicone to lock it in there which I thought would be risky but Eberg has forgotten more info than I know so I would be OK with that as a solution.