That light IS something to worry about, liability-wise. If you pop a leak in one of the two hydraulic systems, the valve in that switch trips and turns the light on to let you know while there is still some fluid left and you have a better chance to safely stop the car. Unplugging the switch sets the shop owner up for a lawsuit in the event of a crash, even if it was the other guy's fault. You can also be found partially at fault when a sharp lawyer points out to the jury that the warning light was on and someone disabled it.
The problem is that pressure-differential valve is spring-loaded on every other brand of car except Fords. When there are equal brake fluid pressures in the front and rear hydraulic systems, that valve stays centered and the switch stays off. During a normal front disc brake job, once the new pads are installed, the pistons must be pumped out of the calipers until the pads make contact with the rotors. No fluid pressure will be developed until that happens. In the meantime the rear shoes are building pressure. It's real easy to prevent the valve from tripping by simply never pushing the brake pedal more than half way to the floor. (Doing so will usually damage older master cylinders too). Since Ford valves are not spring-loaded to reset automatically, you must prevent pressure buildup in the other half of the system, then push the pedal very slowly until the valve centers and the light goes out. The easiest way to do that is with a helper pushing the pedal while you crack open the steel line nut at the master cylinder's smaller reservoir. Open the line nut about a quarter turn, then have your helper push the pedal so slowly it takes about 20 seconds to go half way to the floor. He must stop as soon as the light goes out and wait for you to tighten the nut, THEN he can release the pedal. Releasing it too soon will draw air into the line.
This procedure can be REAL frustrating because the valve can suddenly pop the other way and you might not know it. Then you have to open the front hydraulic circuit again. Because the valve can stick, then pop unexpectedly, you can chase it back and forth all day long!
It is safe to unscrew the switch while you do this. You'll be able to see the valve move as your helper presses the pedal. Sometimes you can poke a pick or thin screwdriver down there. Hold light pressure on it so it drops into the groove when the valve moves to the centered position. They also make a tool to screw into that opening before starting brake work. That tool holds the valve centered until all the work is done.
Wednesday, January 5th, 2011 AT 8:19 AM