This Weatherhead combination brake valve was removed from a 1978 Chevrolet Corvette. According to my parts catalogs the original applications include all 1978 - 1982 Chevrolet Corvettes equipped with 4-wheel disk brakes. The original GM part number for this valve is 1257208. These instructions can also be used to rebuild the 339780 Weatherhead combination brake valve used on 1974 - 1977 Corvettes.
You can click on the pictures below to see larger versions of the images.
This document describes how to rebuild the valve using a new seal kit available from Muscle Car Research LLC. Tools needed:
The seal kit provided by Muscle Car Research LLC includes the following parts:
Weep hole rubber sealing disk
Weep hole rubber sealing disk retaining ring
Arrange your valve, tools, and rebuild kit on a clean work surface. Let's get started! The first step is to inspect the valve and make sure that the exterior is undamaged. Check the port threads for damage. If any of the port threads are stripped or damaged you're better off finding another valve. If your valve passes the exterior inspection you're ready to disassemble it and inspect the internal parts. Disassemble the valve as described in our 1978 - 1982 GM 1257208 Combination Valve Autopsy article.
Clean the parts by soaking in full strength ammonia (overnight is fine), flush with soap and water, dry thoroughly, and polish with the dremel tool and steel wool. Ammonia will remove color from the proportioning valve spring so you may wish to wash that by hand without soaking. Clean the insides of the valve (use the bore brush to clean the rod bore) and make sure that the sealing surfaces are perfectly clean - any dirty residue that lingers in here can cause a leak!
Be especially careful when laying out the new o-rings. Three of them are very similar in size, but if you look closely you'll see that the sizes do vary slightly. From largest to smallest they are for use on the large end (the end with the brass ring) of the pressure differential piston, the large end of the proportioning valve piston, and the small end of the pressure differential piston.
Start by reassembling the pressure differential piston and the pressure differential switch. Lubricate the seals with clean brake fluid and slide them into place using the dental pick.
Here are the completed parts. Note that the o-ring on the large end of the piston fits between the brass ring and the steel washer.
Install the piston in the valve body, small end first. It helps to give the o-rings another light coat of clean brake fluid. Push the piston in just far enough for you to see the grooved area when you look through the switch hole on top of the valve. You need to be able to install the switch in such a way that the contact terminal DOES NOT touch the piston.
Screw the switch into the valve body. Test for continuity between the exposed terminal and the valve body. If you have continuity you need to remove the switch and the piston and re-align the grooved area with the hole in the body.
With the switch in place you can start to work on the proportioning valve. Install the steel washer and spring.
Lubricate the o-rings and install them.
Insert the piston into the proportioning valve body. The large end goes in first. Another coat of brake fluid on the seals will help it slide in easily.
Install the weep hole seal and press the retaining ring into place using a nail set or other appropriate tool; it should deform slightly as you press it down. Lubricate and install the o-ring.
Press the cap into place using your bench vise. Just press far enough to ensure that there's no space between the cap and the valve body.
Screw the proportioning valve into the open end of the combination valve body. You only need to tighten it enough to make sure that it's fully seated.
With the proportioning valve installed your combination valve is fully reassembled!
Now you can reinstall the valve in the car and reconnect the brake lines to the valve. Start each line by hand as best you can to ensure that the fittings are threading in straight and smooth. Be careful - it's easy to cross-thread the fittings and damage the threads. Tighten each fitting with a flare nut wrench. You may need to tighten, loosen, and retighten each fitting multiple times to obtain a leak-free seal. Add brake fluid, bleed the brakes, and check for leaks.