lawsoncl said:
Minor nitpik. The light is triggered when the prop valve sees a significant pressure difference between the front and rear circuits.
Theres a Piston in the Proportioning Valve, really called the Combination Valve because it also contains this safety item. The piston has one brake circuit on one side, and the other circuit on the other side, before the pressure reducer. When you press the brake pedal and make pressure with the Master Cylinder (MC), both pistons in the MC should make equal pressure, and equal pressure for both circuits and thus the piston gets equal pressure on each side and does NOT move.
When there is a signficant difference in pressure on one circuit compared to the other, there is more pressure on one side of the little piston and it moved by the unequal pressure. The piston is connected to a little switch that lights the Brake Light if the piston is moved by the unequal pressure.
http://auto.howstuffworks.com/master-brake.htm
Obviously a leak in one of the brake circuits would cause a pressure drop and that would cause the difference in pressure that moves the piston in the combination valve and light the light. BUT, as mentioned, a blown seal in the Master Cylinder, could cause one of the two pistons to NOT make much pressure and give you the same symptoms and light, BUT NOT lose any fluid out of the system.
A lot of air in system could cause the same, but air usually doesn't get into the system unless you suffer a leak, open the lines or run the reservour dry; so you should have had the light immediately after one of those events. Air just doesn't get into a properly working brake system all on its own, something has to go wrong or someone has to have been working on the brakes to get it in there.