I know it's an older thread, but found it when searching, and the information above helped. Adding additional details for the next guy.
Had a bad wheel cylinder on dad's '86 K20 suburban. He drove it a few times with low fluid in the rear circuit. Got it all repaired, and bled, and still had the brake light. Verified it wasn't the parking brake and WAS the proportioning valve by unplugging it at the frame rail.
Pulled the "reset" rubber plug off of the end of the valve and used pliers to manually shift it in(truck off/brakes not applied). The spool moved in, but left a small silver pin sticking out. So I figured I had ruined it.
Had dad start truck and pump brakes. The spool shifted back to where it had been(or close) and the silver pin was extending out of the top with each application of the brakes, and the light went off.
The fluid in the system was nasty, and we had flushed about a quart of DOT4 through before all of this, thinking that was causing the light(air somewhere). I think we had an off center issue as Imiceman44 noted above. Moving the valve freed it up and it returned to center.
Thanks again CK5!
Had a bad wheel cylinder on dad's '86 K20 suburban. He drove it a few times with low fluid in the rear circuit. Got it all repaired, and bled, and still had the brake light. Verified it wasn't the parking brake and WAS the proportioning valve by unplugging it at the frame rail.
Pulled the "reset" rubber plug off of the end of the valve and used pliers to manually shift it in(truck off/brakes not applied). The spool moved in, but left a small silver pin sticking out. So I figured I had ruined it.
Had dad start truck and pump brakes. The spool shifted back to where it had been(or close) and the silver pin was extending out of the top with each application of the brakes, and the light went off.
The fluid in the system was nasty, and we had flushed about a quart of DOT4 through before all of this, thinking that was causing the light(air somewhere). I think we had an off center issue as Imiceman44 noted above. Moving the valve freed it up and it returned to center.
Thanks again CK5!

