This Kelsey-Hayes proportioning valve (sometimes also called a "rear pressure regulating valve") was allegedly removed from a 1968 Yenko Camaro. I believe that the same type of valve was also used on other GM vehicles, 1967 - 1969 Ford Galaxie and Thunderbird (car lines A and S), 1967 - 1969 full-sized Mercury and Lincoln (car lines A, D, and K), 1968 - 1970 Mopars (car lines B, J, L, R, V, and W), and the 1968 - 1970 AMC American, AMX, and Javelin. I believe the original GM part number for this valve is 3908326, the Ford part number is C7AZ-2B091-A, the Mopar part number is 2852 045, and the AMC part numbers are 319 0928 or 319 3798, with the significant difference being in the physical characteristics of the internal spring. The folks at the Camaro Research Group have an excellent article that describes their research into the 1967 - 1969 Camaro applications for this valve. Ford also used this valve as a later service replacement for the cylindrical C7OZ-2B091-B proportioning valve.
Note the "157 - 8" that's stamped into the valve body. That's a date code. I believe the "157" describes the day of the year (out of 365 possible days; June 6) and the "8" describes the year (1968). Parts like this are often date-coded to describe when they were cast or assembled.
I've been thinking about expanding my line of brake valve rebuild kits to include kits for GM products, and this valve looked similar to the Kelsey-Hayes valves that I'm already familiar with. I decided to disassemble it to see if it's rebuildable. Follow along as I tear it down! Tools needed:
Start by removing the plug from the end of the valve body. Note the o-ring on the plug. Look inside the valve and you'll see another seal and one end of a green piston. Both seals will be needed in a rebuild kit. Remove the piston seal with your dental pick.
When you remove the seal you'll find a thin washer - remove it. There's a snap ring under the washer. Remove it with your snap ring pliers. Be careful, though, because the snap ring is being used to compress a spring.
With the snap ring out of the way you can remove the spring and the piston with your needle nose pliers. Note the third seal on the end of the piston - it will often remain inside the valve when you remove the piston. If you remove the piston and the seal doesn't come out with it you'll need to remove the seal separately with your dental pick. Here's what all of the internal parts look like.