1970 GM Pressure Differential Valve Autopsy

I recently removed this pressure differential valve from the left front frame rail of a 1970 Chevrolet Impala. I believe the original part number is 3980761. Complete rebuild instructions and a rebuild kit are available to help you rebuild this valve.

These are the tools you'll need to disassemble the valve:

  • 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
  • Large slotted screwdriver
  • 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
  • Impact driver (optional)
  • Penetrating oil

Step 1: Remove the external fittings and pressure differential switch using the sockets and wrenches.

I discovered the first o-ring seal when I removed the pressure differential switch. A replacement for this seal is included in the 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 are included in the kit.

The bracket is attached to the body using two 10-24 x 3/8" slotted round head machine screws with #10 lock washers. I am unsure of the original finish. If rusty, these screws can break or strip if you try to remove them with a standard screwdriver. Soak them in penetrating oil and use an impact driver to loosen them before trying to remove them.