I can help anyone that's interested in rebuilding their 339780 (1974 - 1977 Corvette) combination brake valve and the later 1978 - 1982 1257208 valve. I recently updated and posted some new information on my web site that describes how to disassemble and rebuild the valves using a 1257208 valve as an example. Here's the disassembly article:
https://www.musclecarresearch.com/gm-1257208-autopsy
The rebuild article:
https://www.musclecarresearch.com/gm-1257208-rebuild
A short video that describes how to disassemble the proportioning valve:
https://www.musclecarresearch.com/co...ve-disassembly
The autopsy article describes the bits needed to make a tool to open up the proportioning valve. I can provide them if you don't want to source them yourself. Here's the tool ($9 plus shipping):
https://www.musclecarresearch.com/br...ug-pusher-tool
and the rebuild kit ($10 plus shipping):
https://www.musclecarresearch.com/gm-pd-valve-kit-3
Can anyone recommend a good 'vette-focused forum that would welcome this kind of information? Most of the forums I've seen require a hefty "supporting vendor" fee to post an ad like this. I don't make enough money off this stuff to pay several hundred dollars in advertising fees every year.