1990 GMC Sierra Dash Brake light ON

Which light came on? Red brake light? ABS? both? or service engine?

The only light on is the red brake light. It is on constantly. I don't believe this truck even has ABS, but not sure. I really think it's the same light that comes on when a brake line "blows", sending the switch to one side or the other. The key is it came on right after front brake lines were replaced.

Check the fluid level in the master, and check the parking brake lever switch. If they are ok, check for any leaks, and that DOT 3 brake fluid is the only thing that was added! You can tell if the rubber has swelled on the master cyl, cap!

PLEASE, are you reading my question thoroughly? I've already checked all the items you've mentioned. No leaks, parking brake switch OK, master full. OH, I have checked and found that the light DOES get slightly brighter when P-brake is applied. Hmmm...
Thanks again
Zip

If all these where checked, it's possible the proportioning valve is stuck off center, jack up the rear of the truck press brake pedal, do the rear brakes hold? The other thing to do is grab an assistant to help bleed the front calipers, you will need to press and hold the button on the front of the prop valve while you bleed it. Sorry about the confusion!

Wow!! Thanks for the quick reply. I was just heading outside to bleed the prop. valve when I got your reply. I'll let you know how it works out.
Zip

OK