1972 GM Pressure Differential Valve Autopsy

This pressure differential valve was allegedly removed from a 1972 Chevrolet Nova. I bought it through eBay for autopsy purposes. It looks similar to another valve that I found on a 1970 Chevrolet Impala, but the differences in the switch and valve body had me wondering if there are any differences in the internal parts. I believe the original part number is 3980762.

I've been thinking about expanding my line of brake valve rebuild kits to include kits for GM products, and this valve looked similar to the Wagner valves that I'm already familiar with. I decided to disassemble it to see if it's rebuildable. Follow along as I tear it down!  Tools needed:

  • 5/16" open end wrench
  • 3/8" socket or box wrench
  • 9/16" socket or box wrench
  • 5/8" box wrench
  • 6-32 threading tap
  • Dental pick (or other small, pointed tool)
  • 6-32 x 1" machine screw
  • 6-32 nut
  • Flat washer with 1" OD and 1/8" ID

Step 1: Remove the external fittings and pressure differential switch using the sockets and wrenches. The switch post can be removed with a little pulling.

I discovered the first o-ring seal when I removed the pressure differential switch post. A replacement for this seal will need to be included in a rebuild kit.

OK, now what? There are no more fittings to be removed, but if you look into the hole where the pressure differential switch was you can see a piston inside the valve body.  How can it be removed?  The trick is in knowing that one or both of the brass tube seats at the end of the valve can be removed to allow access to the piston.  If you look closely at both seats you'll see that at least one of them is pressed into place.  Look closely at the outer edges of the seat and you'll see that it's not cast as part of the valve body.

So how can the seat be removed?  The best way to remove the seat without damaging it is to tap the opening with a 6-32 tap, thread a 6-32 x 1" puller screw into the hole, and remove the seat with a few twists of a wrench.

Here's the puller I made using a 6-32 x 1" stainless steel machine screw, a 6-32 nut, and a 1" washer. Muscle Car Research sells a puller tool if you'd rather not source the parts yourself.

Hold the screw in place with a screwdriver.  Turn the nut clockwise with an open end wrench.  The seat will be removed cleanly and can be pressed in again when it's time to reassemble the valve.

After removing the seat you'll be able to reach inside the valve with your dental pick to remove the piston and springs that surround it

Note the two springs that work to keep the piston 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.  There are o-ring seals at each end of the piston; both of these seals will need to be included in a kit.

Are you interested in a rebuild kit for a valve like this? I'd like to hear from you. Please contact me to let me know that you're interested. It would also be helpful if you could tell me what you think a reasonable cost would be for a kit to rebuild this type of valve.