1969 GM 3956727 Pressure Differential Switch Autopsy

I recently found this brass pressure differential warning switch on eBay. It was originally used used on a 1969 Chevrolet Corvette; the original part number is 3956727. It's purpose is to detect differences in front-to-rear brake system fluid pressure. 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 a date code of "3-69" (March 1969). The bracket is marked with "PATENT 3369090 3374322" and a Weatherhead logo. I decided to disassemble it to see if it's rebuildable. Follow along as I tear it down!  Tools needed:

  • 5/8" box or open end wrench
  • 5/16" box or open end wrench
  • Dental pick (or other small, pointed tool)
  • Nail set (or other tool to push with)
  • 6-32 threading tap
  • 6-32 x 1" machine screw
  • 6-32 nut
  • Flat washer with 1" OD and 1/8" ID

Step 1: Remove the pressure differential switch using the 5/8" wrench.

I discovered the first two o-ring seals when I removed the pressure differential switch. Replacements for these seals will need to be included in a rebuild kit.

The internal parts can be removed by first removing the brass tube seats that are pressed into the fluid ports on each end of the body. The seats can be removed by tapping them with the 6-32 threading tap, thread a 6-32 x 1" puller screw into the hole, and remove the seat with a few twists of a wrench. Start by tapping the opening in the seat.

You can create a puller tool using a 6-32 x 1" machine screw, a 6-32 nut, and a flat washer with 1" OD and 1/8" ID. Thread the nut onto the screw. Insert the screw and nut into the hole in the washer. Thread the screw into the hole you just tapped in the seat.

Turn the nut clockwise with the 5/16" wrench.  The seat will be removed cleanly. Muscle Car Research sells a puller tool if you'd rather not source the parts yourself. Though the seats can be reused if they're not damaged during removal, I prefer to include new seats in my rebuild kits.

Repeat the process on the other side of the valve to remove the second tube seat. After removing the seats you'll be able to reach inside the body with your dental pick to remove a pair of springs. Now you can finish removing the internal parts by gently tapping them out of the body with a nail set, screwdriver, or other tool. You'll find a plunger between two brass cups that are sealed with o-rings.

Note the two springs that work to keep the plunger centered when the brake fluid pressure is even between front and rear.  These springs are intact and possibly reusable (if a little rusty), but I normally include new stainless steel springs in my rebuild kits for peace of mind.  The o-ring seals on each cup will need to be included in a kit.