91 Gmc brake light comes on after 40 mph

My 91 gmc pickup has a problem with the brake light on the dash coming on as soon as the truck reaches 40 mph. If I keep the speed under 40 the light
never comes on. I checked for codes and there are none. I tested the speed sensor and it seems fine. Today I replaced the Electronic Brake Control Module with no luck. Does the speed sensor in the transfer case also control the speedometer in the dash? My speedo works fine. Does anyone have any idea what the problem could be?
Thanks, Mariano
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

as
and
no
Try unplugging the proportioning valve sender. It's a little yellow, single wire plug just below the master cylinder. The prop. valve sender sits in the top of the aluminum prop. valve.
Doc

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.