I found this 1973 A-body combination brake valve on eBay. I believe the original part number is 3549947. Original applications include 1973 through 1976 Plymouth Duster, Scamp, Valiant, Dodge Dart, and Demon with front disk and rear drum brakes. The same type of valve was used on other Mopar vehicles from January 1, 1970 - 1976. It's purpose is to detect differences in front-to-rear brake system fluid pressure and to control fluid pressure to the rear drum brakes. When a pressure difference exists, a plunger inside the brass body shifts position and makes contact with a switch, completing a path to ground and illuminating a warning lamp.
The brass body is marked with the logo of the Weatherhead Corporation and a date code. The date code on this valve is "269 2", which translates to day 269 of 1972, or September 25, 1972.
Kelsey-Hayes also manufactured this type of valve, though as we'll see later the internal parts are slightly different. The brass body of a Kelsey-Hayes valve is marked "K/H" and also includes a date code. An example date code of "212 0" (July 31, 1970) is shown below.
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 at the end of the proportioning valve using the 3/4" socket or box wrench. Remove the seals, spring, and piston.
The spring found in these valves is usually resistant to corrosion and may be reusable. The three seals (o-ring, u-cup, and piston seal) will need to be included in a rebuild kit. The Kelsey-Hayes proportioning valve assembly is identical.
Now remove the switch assembly using the 5/8" box wrench. You'll find another o-ring seal that will need to be included in a rebuild kit. The Kelsey-Hayes switch assembly is identical.
Next, remove the large brass adapter from the end of the pressure differential valve using the 11/16" (Weatherhead) or 3/4" (Kelsey-Hayes) socket or box wrench. Remove the large piston assembly.
Note the copper crush washer and o-ring seal. Both will need to be included in a rebuild kit. If you now look down into the bore of the pressure differential valve you should be able to see a small brass spacer and the end of another piston. You can extract the spacer using your dental pick. You may be able to extract the piston by tapping the valve body against a firm surface, such as your work bench. If that doesn't work you'll need to apply air or hydraulic pressure to the port behind the piston.
With the small piston removed you'll find the last two o-ring seals.
The Kelsey-Hayes pressure differential piston assembly is a little different. When you remove the brass adapter you'll find a spring and a slightly different copper washer. The spring is pressed on to the end of a single piston. Remove the spring and then remove the piston with your needle nose pliers.
With the piston removed (note the three o-ring seals) you should be able to see a second, smaller spring in the bottom of the bore. Turn the valve over, tap it on your work bench, and the spring should fall out easily.
Mopar service manuals include a cut-away drawing of the Kelsey-Hayes valve. Note that the Weatherhead valve does not include the small spring (noted in red below) that's used with the Kelsey-Hayes valve.