Proportioning valve questions

I have a used weatherhead proportioning valve from the donor car that my spindles and caliper brackets came from. I was looking it over as to whether or not I could use/rebuild it (I found a place with kits with new seals for $15), and the proportioning piston is in pretty tough shape (see pics). so far I've come up empty on finding a replacement piston, so I'm looking at buying a new valve. Does anyone know if there is any significant differences between the valves different vendors sell? Looking at Summit Racing, Inline Tube, Dr. Diff, and various sellers on ebay, I see them ranging from $45-150+ for what appears to be the same part. Ralistically are these all coming out of the same factory in China and everyone is just reselling it, or is there an actual quality difference between the $45 ebay offering, the $68 inline tube one, $75 summit racing one, the $85 dr. diff one, and the $163 Right Stuff part?

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