I recently purchased from a popular on-line Z-car supplier a rebuild kit for the rear brake proportioning valve on a '71 240.  The problem is that rebuilding of the valve assembly appears all but impossible.  Somehow a spring has to be compressed about half way into a cylinder and held in place while a spring clip is pinched, inserted and pressed against the top of the spring.  I have read some comments on this forum that no, it is not possible to rebuild, but then why would a reputable supplier