Brake combination valve

[Chuck71RS]

With the engine off, parking brake not engaged, key in "run" the brake warning light comes on.

Remove the wire from the combination valve switch terminal, brake light goes off
Push the parking brake pedal and the light comes on
Release the brake and the light goes off

From a Twisted Metal posting, when one side of the switch piston has more pressure then the other, it moves into contact with the pin at the end of the switch terminal, closing the circuit which lights the warning light

What do I need to do?

 

[budro6968]

When you start up does the light go out?

 

[jamieg285]

First thing will be to work out why the valve has gone to the side. There'll be a problem in either the front or rear brake lines - either a blockage causing high pressure, or a leak and low pressure.
Work out which and fix it.

After that, stamp and release the pedal a few times - it can shock the valve back to the middle.

 

[Chuck71RS]

When you start up does the light go out?

Light stays on. valve terminal had not been attached in three years when the condition was realized. Took it off, got busy with other rebuild and forgot about the problem

 

[Twisted_Metal]

If the switch piston is off-center and the sensor pin is grounding... You can't tell which way it has moved.
It's a bit of trial and error to resolve the issue.

With the key on and the light illuminated...

Get a helper and follow the procedure in this video.
GM Proportioning Valve Reset - YouTube

You may want to put a piece of vacuum line on the bleeder (other end in a bottle) to prevent a brake fluid mess.

When the light goes out, you've got it reset.

 

[Chuck71RS]

Success. The high pressure side was the front brakes. With a neighbor working the brake pedal I worked the front LH brake bleeder. Working this bleeder turned off the warning light. Bleeding all four brakes, poured almost 16 oz of brake fluid into the master cylinder with most in the rear section. A lot of air bubbles came out of the two rear bleeders

 

[pomartin]

Congrats Now take her for aspin

 

[rode2nowhere]

Well mine has been on for over 10 years, guess that's next. ( brake light on )