1967 - 1975 GM Pressure Differential Valve Rebuild

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This document describes how to rebuild the 1967 - 1975 GM brake pressure differential valve using a new seal and spring kit available from Muscle Car Research LLC.  Tools needed:

  • Wire wheel or bead blaster
  • 5/8" box wrench
  • 9/16" brake line fitting
  • Liquid ammonia
  • Rubbing alcohol
  • Clean brake fluid
  • .50 caliber bore brush
  • Ball peen hammer
  • Nail set or thin screwdriver
  • Penetrating oil

The rebuild kit provided by Muscle Car Research LLC includes the following parts:

  • Springs
  • Piston o-ring seals
  • Contact terminal o-ring seal
  • Brass tube seat
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 your valve is attached to a mounting bracket, make sure the body is securely attached to the bracket. A loose mounting bracket on a brass valve can be tightened up by lightly tapping the brass mounting tab with a ball peen hammer. Cast iron valve brackets are typically attached with screws and lock washers that are probably best replaced. 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 1970 GM Pressure Differential Valve Autopsy or 1972 GM Pressure Differential Valve Autopsy articles. Iron parts can be bead blasted. Soak the brass parts for a few hours in a container of liquid ammonia. Ammonia does a great job of softening tarnish on brass, but be careful of the smell! Remove the parts, flush with water, and polish with steel wool. Clean the bore of the valve with the bore brush and make sure that the bore is perfectly clean - any dirty residue that lingers in here can cause a leak! Finish cleaning the parts using alcohol and cotton swabs.
 
The bracket can be cleaned with a wire wheel or by bead blasting. Rinse with alcohol when finished. Lay out the clean parts and your rebuild kit.
 
 
Install the new o-ring seals on piston. It helps to lubricate the seals with clean brake fluid.

Install the first spring, the piston, and then the second spring. You will need to apply a little pressure with a nail set or thin screwdriver to insert the piston. Lubricate the o-rings again to avoid damaging them as you install them. The second spring should be below the beveled edge that mates with the brass tube seat.

Lay the brass tube seat in place and make sure that it's not cocked.

Press the seat into place using a brake line fitting. Tighten it well to ensure that it's seated.

Turn your attention to the terminal. GM used two types on these valves. One type can be identified by the black plastic insulator that's visible above the hex nut. This terminal can be disassembled by removing the screw from the bottom of the terminal assembly. Clean the plastic parts with soap and water and the metal parts with a wire wheel or by bead blasting. Do not lose any of the small plastic parts! They are needed to insulate the contact from the valve body.

The second type can be identified by the steel body (no black plastic) above the hex nut.

This terminal also uses plastic insulators to shield the contact from the valve body. Note that the two switches use different o-ring seals. Be sure to correctly identify the type of terminal you're working with when ordering a rebuild kit.

Screw the terminal into the body of the valve - and you're done!

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 soft threads on a brass valve body. 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.

The bracket used on this valve was originally plated with a zinc dichromate or yellow cadmium finish. If you're rebuilding the valve for use on a concours show car you will want to remove the bracket from the valve. On a cast iron valve the bracket can be removed by removing the two 10-24 x 3/8" screws that secure it to the body - just be careful when working with rusty screws that can break when you try to remove them! It may help to soak the screws with a quality penetrating oil and then loosen them a bit with an impact driver before trying to turn them with a screwdriver. On a brass valve the bracket can be removed by carefully reforming the staked brass tab that holds the bracket in place. After plating, the bracket can be reinstalled by restaking the brass tab with a ball peen hammer.

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