Brake light 1989 Cheyene

[ChevyTech]

It sounds like the RWAL system is turning the dash brake light on.
RWAL stands for REAR WHEEL ANTILOCK. RWAL was standard equipment starting in 1988 on full size trucks.

1988 and newer trucks of that body style that do not have 4 wheel ABS have RWAL.

There are three brightness levels of the dash brake indicator light on RWAL equipped trucks.

Brightest – park brake
Medium – Combination valve (aka metering & proportioning valve)
Dim – RWAL System

You can unplug the wire on the Combination valve to see if it is the cause on the light coming on.

The RWAL system can store trouble codes, but it is only capable of storing one code at a time.

You can retrieve RWAL codes without a scan tool.

To retrieve RWAL codes connect a jumper wire between terminal H and terminal A of the ALDL connector with the key on (engine not running). Watch the dash brake lamp and count the blinks. Sometimes RWAL system send out a phantom blink before the code blinks start. It can take up to 20 seconds for the code to start coming out.

Checking the code with this method can set a false code 9. Checking for a code when no code is present sets a code 9.

You can clear the code by removing the fuse for 10 seconds then reinstalling it. The fuse should be labeled STOP/HAZARD or BRAKE. Removing the RWAL fuse will not clear the code.

If you get one long flash and 5 shorter flashes that is a code 6

On trucks built after Aug 1994 a scanner is required to retrieve codes 6,9,10

This site tells how to read your codes.
http://autorepair.about.com/library/faqs/bl929g.htm

This site has general information about RWAL.
http://www.babcox.com/editorial/tr/tr89850.htm

On this system it is usually best to: Check the code, clear the code, and drive the truck and see if the code returns.

Let me know what you find.