I did a lot of checking into these in the spring trying to bleed my brakes before the first car show of the season. There is nothing inside that is rocket science, an O ring, spring, metal washer, snap ring, spool valve, and a cup washer. I have taken a couple of one’s apart and the seals usually swell up right after disassembly.
The repops on ebay are not built like the original ones either inside or outside (surprise). I did find this place that would rebuild the valve with the correct seals for brake fluid.
http://www.resleeve.com/index.htm I have not sent mine in yet, but will do it over the winter.
Mike