1970 GM 3983040 Pressure Distribution Switch Rebuild

This document describes how to rebuild the 1970 GM 3983040 brake pressure distribution switch using a new seal and spring kit available from Muscle Car Research LLC.  Tools needed:

  • 9/16" and 1/2" socket or box wrench
  • 5/8" box wrench
  • 9/16" and 1/2" brake line adapter, plug, or fitting
  • Brass and copper cleaner
  • Fine steel wool
  • Small round file
  • Clean brake fluid
  • .45 caliber bore brush
  • Long thin pin punch (or other tool to push with)
  • Ball peen hammer
  • Liquid ammonia

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

  • Springs
  • O-ring seals
  • Tube seats

Arrange your valve, tools, and rebuild kit on a clean work surface. Let's get started! The first step is to inspect the part and make sure that the exterior is undamaged. Check the port threads for damage. Make sure the body is securely attached to the mounting bracket. A loose mounting bracket can be tightened up by lightly tapping the brass mounting tab with a ball peen hammer. If any of the brass port threads are stripped or damaged you're better off finding another switch. If your switch passes the exterior inspection you're ready to disassemble it and inspect the internal parts.

Disassemble the switch as described in our 1970 GM 3983040 Pressure Distribution Switch Autopsy article. Soak the internal metal 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! Ammonia can also damage the plating on the bracket, so don't soak the bracket if you wish to preserve the original plating. Clean the switch itself with brass cleaner. One brand I've found to work well is "Twinkle". It's a paste that can be rubbed on and rinsed off with water. 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! Rinse the parts thoroughly and wipe them dry.

A rusty bracket can be cleaned with a wire wheel or by bead blasting, but note that this will remove the original plating. Wash the part and dry it thoroughly when finished. Lay out the clean parts and your rebuild kit.

Install the new o-ring seals on the large piston. It helps to lubricate the seals with clean brake fluid. Note the orientation of the piston relative to the other parts. The hole in the end of the piston should face the small rubber plug in the steel cap.

Insert the large piston into the "bracket" end of the switch. The hole in the end of the piston should be facing you. Gently push it down until it seats in the bore.

Install the steel cup and plug into the same end. The plug should line up with the hole in the piston.

Install the short spring into the same end. It should fit between the arms of the steel cup.

Install one of the tube seats. Press everything down just far enough to get the tube seat straight. If your original tube seat includes notches that align with the fluid ports on the valve body, you should add those notches using a small file. You might also need to grind some material from the bottom of the seat to match the height of the original tube seat.

Press the tube seat into place using the 9/16" brake line adapter, plug, or fitting. If you don't have one handy you should be able to find these fittings at your local full service auto parts store. Tighten the adapter only until it bottoms out.

​Remove the adapter and inspect the tube seat to make sure it has been properly installed.

Turn your attention to the other end of the switch. Install the last of the smaller o-ring seals on the small piston. Lubricate the seal with clean brake fluid.

Install the spacer into the end of the switch. You should be able to see it by looking through the terminal port.

Install the short piston with the open end facing you. The second, longer spring needs to fit in the open end of this piston.

Install the second spring. Make sure it fits into the open end of the small piston!

Install the second tube seat. You might find it easier to remove the spring, push the spring into the "back" side of the tube seat, and then insert combined the spring/tube seat into the end of the switch. You might need to modify this tube seat, too.

Press the tube seat into place using the 1/2" brake line adapter, plug, or fitting.

Remove the adapter and inspect the tube seat to make sure it has been properly installed.

Almost finished! Lubricate the large with clean brake fluid o-ring and install it on the internal end of the terminal.

Place the steel plate on the switch over the termianl port and insert the terminal through the plate and into the switch. Finished!

Now you can reinstall the switch in the car and reconnect the brake lines to the switch. 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 brass 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.

The bracket used on this switch was originally plated with a zinc dichromate or yellow cadmium finish. If you're rebuilding the tee for use on a concours show car you will want to remove the bracket from the tee 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. You might also be able to replate the bracket with the valve in place. Any plating that gets on the brass body of the switch can be removed with steel wool.