I'm rebuilding a master cylinder for my '38 Master HB, and ordered a rebuild kit from The Filling Station, kit p/n FS-9. The piston in the kit is different from the original one. The new piston features a spring check valve disc on the face of the pressure side of the piston. The center of the piston face has a small "nub" and the disc is pressed onto it for retention. The thing that I don't like is that the flat back face of the "rubber" cup should lay flat on the piston face. With the new piston, the nub prevents the back of the cup from being flat. For sure, the return spring and brake fluid pressure will flatten the cup over the nub, but then the rubber will be stressed where the nub is pressed into it. To me, this looks like a way to cause the rubber to eventually fail.

Does anyone have knowledge about the design of the new piston in the kit? Should there be a different rubber cup to work with it? I'm somewhat inclined to just remove the spring disc and the nub on the piston, to make it like the original design (as also shown in the '38 manual). Steve at Filling Station says they've sold a lot of the kits and haven't received any complaints. He supposes that the nub pressing into the rubber is an intentional design feature. My work experience has taught me that parts suppliers are quite capable of coming up with design flaws, so I'm skeptical.

I'll appreciate your thoughts and experience.