OK, I have knowledge of all different brands and models of vehicles. Problem is, keeping them all straight.
So, some of this may be wrong.....
Depending on the year, they are right in that either the switch on the emergency brake pedal or the proportioning valve will turn on that light.
Easy way to make sure. With the key on, and the brake light out, lightly press the E-brake and see if the light comes on.
A weak return spring can let the pedal move far enough due to its weight to turn the light on sometimes.
Quick test for that is to disconnect that switch and see if the problem stops.
Some vehicles also have a fluid level switch that will turn on a light. But you will see some wires going to the master cylinder if you have that switch.
The proportioning valve switch is a spring loaded piston with a reduced center that a spring loaded plunger sits in.
When you step on the brakes, the piston sees pressure from both the front and back brake lines.
As long as they are about equal in pressure and flow, the piston stays centered. If there is a big difference like with a burst line or bubbles on one side, it moves over and the plunger pushes up and turns on the light.
Again easy to check. When the light turns on, stop the truck, and with the light still on, unplug the wire on the valve. If the light goes off, then that is the switch that is turning it on.
Now, working from memory, the bigger chamber of the MC is for the front brakes, since the caliper diameter is about 3 inches across, and the rear cylinder is about 3/4 inch.
Takes lots more fluid.
Plus, if the rear adjusters are working, then the rear cylinder take the same amount of fluid each time. Since the adjusters move the other end of the shoes apart and the cylinder pistons should move the same amount.
The front calipers take much more fluid per amount of travel since they are so much bigger than the rear.
Plus, when the pads are new, the pistons are almost all the way in. As they wear, they move out and stay out.
So more fluid is required.
Again its easy to check. Take the top off the MC, have some one watch it, while you stick a prybar in the inspection hole of the caliper and pry the pad back toward the piston pushing the piston back slightly.
As soon as it moves, there will be a little geyser in whichever chamber is hooked to the front.
Don't move it much, or you will get a light when you hit the brakes next time.
Next, hit the brakes a few times to get everything bottomed out. Release the brakes, have someone watch the fluid with the top off.
Press the brakes hard, hold for a second and then let them off.
If you have air in a line, you will see a gusher in that chamber as the air expands and pushes the fluid back into the MC.
You might see a small movement with no air, but if there is air its fairly obvious.
Next, if you have air in any lines, make a note of which line has the has the air. That will help you pinpoint where its coming from.
For instance, if the left front is the one each time, then the problem is between the dividing point and the caliper. So, check the fittings, bleeder screw, and flex line.
If those are good, it might be getting sucked back around the piston seals.
If its in both front lines, then the problem is going to common to both lines. MC, prop. valve, fittings.
If you are getting air each time, and its not getting in anywhere, then you don't need to do any special bleeding techniques. You will eventually get it all out. If its getting in after bleeding, then nothing will get it all out until you fix the problem.
The only ones I know that actually give an error through the brake light are the 90 and 91's with ABS.
