Brake Warning Light

[motormayhem]

I noticed every once in a while when I panic stopped my warning light would come on on the dash for a second and then go off. Recently I changed the front brake lines to longer lines off a 92 2wd chevy. They are brand new, but about 4" longer. Now when I hit the brakes hard but not to the floor the light comes on and I have to let off and hit the brakes again to make the light go off. What does the light indicate? I know it is the proportioning valve being unhappy but what exactly triggers it? Would it have anything to do with having the bigger 14 bolt brakes on the rear of my m1009? The brakes seem to be working ok.

 

[doghead]

Air, bleed them more.

 

[rlwm211]

I would also suggest that you make sure your rear brakes are adjusted
up. If too much fluid goes to one circuit of the brakes, you can trigger the light. ​

Another possibility is that the E-brake pedal is not returning to keep the light off and when you brake and let up and brake the pedal is moving slightly and allowing the light to come on. This happened to my suburban, which has the same cab, and setup as the M1009. ​

Just two cents worth!
RL​

 

[Barrman]

If you stick your head under the front of the truck and look forward at the front most cross member. You will be staring at the brake proportioning valve. It is actually something called a combination valve because it meters how much goes to the front and rear due to the disc/drum combination and if it senses too much fluid going to one end or the other. It has a piston that shuts off flow to that part of the circuit. The manual even tells you that when bleeding the brakes, a special plate has to be bolted onto the combination valve to keep that rod from doing what I just described. The brake warning light comes on when that rod moves.

By pumping your brakes, you compress the air in the lines and the fluid pushes the rod back to the center. Bleeding with a pressure bleeder eliminates this hassle. If you are going to bleed the old fashioned way, let out just a little bit at a time per time pressure is pumped up and don't push too hard with the pedal.

Simple way of figuring out brake problems:

Hard pedal that slowly goes down as you hold it such as at a stop light and fluid is being lost=leak somewhere, look for drips and you will find the problem.

Hard pedal that slowly goes down at a stop light and no fluid is ever lost=bad master cylinder. Rebuild or replace.

Soft or spongy pedal that can be pumped up to hard with 1 or 2 pumps=air in the system

Hard but low pedal that can be pumped up higher with drum brakes somewhere on the vehicle=shoes that need to be adjusted.