Kelsey-Hayes proportioning valve for A body

Now that you jogged my memory, I know you are correct about the RPV being in the master cylinder. I got my terminology and brake parts mixed up! E burg or somebody wrote an article back in the day before Al Gore invented the internet, telling how to remove one from a drum brake master cylinder to convert it to use with front discs. You just remove the brake line from the end of the reservoir you want to use for the discs, run the appropriate size sheet metal screw into the hole the brake fluid comes thru and wiggle that little baffle out. Behind it, you will find a spring that looks like one from a ball point pen with a rubber hickey in it. You throw the spring and rubber hickey away and remove the screw from the seat you pulled out and put the seat back in the hole in the master cylinder and let the fitting on the line push it back in place when you tighten the line down. I tried it once on a 74 Duster I converted to big bolt pattern discs. It worked ok, but I wouldn't do it again.
Well, I agree, you can remove it - sometimes. For me it’s about 50/50 if the brass bit even comes out. Sometimes they get stuck pretty good. Just removed one on the rear of an oem 74 master for a disc brake car and there wasn’t even one there.
the reservoir size is a lot smaller on a drum master which isn’t ideal for a disc application. A caliper compressing in consumes a lot more fluid than a wheel cylinder Does hence the larger size reservoir for the front circuit on a disc brake master.