This Weatherhead brake combination valve was removed from a 1978 Chevrolet Corvette. I bought it through eBay for autopsy purposes. The valve includes both pressure differential (in the middle) and proportioning (at right) components, but no metering valve because a metering valve isn't needed for cars equipped with 4-wheel disk brakes. I've seen similar brass-bodied valves that include a metering valve on the left side for cars equipped with front disk and rear drum brakes.
The original GM part number is 1257208. This valve was used on 1978 - 1982 Corvettes.
This valve is similar to the 339780 valve used on 1974 - 1977 Corvettes. The earlier valve uses a different pressure differential switch, has steel mounting inserts, and a slightly different internal pressure differential piston. The valves are otherwise functionally identical.
The following tools are needed to disassemble the valve:
Remove the external fittings and pressure differential switch using the sockets and wrenches. I discovered the first o-ring seal when I removed the pressure differential switch. A replacement for this seal will need to be included in a rebuild kit. This particular switch is made of nylon; I've seen both metal and nylon switches on the 339780 valve.
Put the valve body in a vice with pads or rags to protect the surface. Remove the proportioning valve assembly and another o-ring seal appears. A replacement for this seal will need to be included in a rebuild kit.
With the proportioning valve removed you should be able to see one end of the pressure differential valve piston in the bore of the valve body. It needs to come out, but there's no easy way to grab it. The easiest way I found to remove it is to block off two of the ports on the left side of the piston (you can use plugs that are made for this purpose or old fittings with flattened brake lines that are sealed shut) and then blow compressed air into the third port. Cover the open end with a thick rag to catch the piston, apply compressed air, and the piston pops out. You'll notice two more o-rings seals on the piston. Both will be needed in a rebuild kit. These o-rings are similar in size, so keep track of their relative position.
So far this looks pretty easy, but what about the proportioning valve? Let's take a closer look at it.
There's a weep hole that's covered with a small rubber disk. The disk is secured with a self-locking internal retaining ring; both should be included in a rebuild kit. The internal parts of this valve are secured with a pressed-in plug that's visible at right in the picture above. The plug can be safely removed using a pusher tool made from a 3/4-20 spindle nut (GM 378137, Dorman 615065, or NAPA 630-1523, for example) and a side-mount external retaining ring (McMaster-Carr 98420A140). Muscle Car Research sells a tool made from these parts if you'd rather not source them yourself.
This is what the tool looks like once installed.
Place the valve in your bench vise with the threaded end pointing up. Thread the nut onto the end of the valve. Push the retaining ring into place. Now unscrew the nut and push it up against the retaining ring until the plug is removed. With the plug removed we can extract the spring and piston from the proportioning valve. Remove the two o-rings from the piston to complete the disassembly. Note that the large o-ring is similar in size to the two o-rings used on the pressure differential piston. Again, keep track of the relative position of this o-ring.
So what do the 339708 valve parts look like? The different parts are pictured below. Compare the pressure differential switch (male metal vs. female nylon), steel mounting inserts, and the black end of the pressure differential piston (rubber stop vs. spring-loaded cap).