Proportioning valve
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
You can test the differential section (the part that turns on the failure light) by opening a bleed screw and applying pressure. It could take 3-400 PSI to trip it so you can't do it with an air hose on the bench. You can remove the switch and differential piston to check the o'ring seals, but the proportioning section (the big nut on the end) is pressed together. I have attached a pic of the prop section disassembled so you can see what's in it. If it were me, I'd replace it since the seals will be flat and prone to leak due to age. The proportioning section could be tested on the car with a couple of 1000 PSI gauges. Install one gauge at a rear brake and one at the master cylinder outlet and apply the brake to about a 1000 psi at the MC. If the proportioning valve is working, you should get about 600 PSI at the rear wheel.
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
I once had a Camaro that I couldn't get a good pedal on even though I had bled the brakes multiple times. Replacing the proportioning valve allowed me to bleed the brakes and get a good pedal. I don't know how to test it but I would say they can go bad.
