Brake warning light

[just plain john]

What triggers the brake warning light, low line pressure? If the fluid is full, would that point to the master cylinder? The brakes seem fine, but the light comes on, sometimes bright, sometimes hard to see.

[SuperSix]

Is this a powermaster, vacuum or hybroboost system?

The brakes seem fine, but the light comes on, sometimes bright, sometimes hard to see.

So - sometimes it illuminates but very dim? THat sounds like an electrical issue, possibly a weak/loose ground.

Here's the circuit diagram for the PM:
http://www.gnttype.org/techarea/images/PwrMstr.gif

Looking at this, the light can be triggered by three things - The combination valve (on frame rail under brake assy), the parking brake switch, or the brake pressure switch on top of the PM.

Master cylinder failure is usually indicated by a soft pedal that will press down more the longer you keep your foot on the pedal.

Assuming this is a Powermaster system - do you notice the brake pressure motor running more than normal?

Powermaster diagnosis:
http://www.gnttype.org/techarea/brakes/PwrMstr.html

[just plain john]

I'm sorry, I guess I should have been more specific. This is not a TR, nor the Powermaster brake system. Just a plain disc/drum vacuum setup on a mid 80's Chevy truck. Pedal seems fine, and does not creep down if depressed.

[SuperSix]

Do the brakes still work as they should? Just he light is coming on?

To get email notifications, you can either set it in your profile to receive email notifications for all threads you post to, or, if you want just notification on the individual thread, when replying (using the Reply button) click "Attachments and other options" under the text box, and click the box next to  "Notify me of replies."