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I was cleaning my engine today (351W) and removed the 2 wire electrical plug(white wires with purple stripe) next to the master cylinder to clean then reinstalled it. Now the "Brake System" light is on in the dash, wasn't on before. I have been reading I will have to bleed the brakes to reset it. Do I do both front brakes or just one side in the front to relieve the air in the system before it goes out? Step by step process would be greatly appreciated, thank you!
 

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Bad connection. or bad switch. Can’t imagine you cleaning anything and moving that shuttle inside.

ex-Global West GM
1991-1995
 

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The wire probably wasn't on completely before. If the switch is still working (who knows after 50 years?) then the little shuttle inside the valve has moved off center and turned the switch on. It moves when you lose pressure in either the front or the rear brake circuits or when you bleed the brakes.

The step by step process is in your shop manual, but you basically need to bleed the fronts and see if the light goes off, and if that doesn't do it bleed the rears. If bleeding both ends doesn't fix it then either the switch is bad or the shuttle is stuck.

You can unscrew the switch and check it, but if the shuttle is stuck that's going to be more work.
 

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The wire probably wasn't on completely before.
Yup that is a definite possibility.
Moving a shuttle that is not centered is a royal PITA. The longer it's been off center, the worse it is to center (usually).
There is a procedure in the Mustang shop manual that can guide you in getting that shuttle back centered. It's an imprecise
"science" and a pretty good test of your patience.....

In the future, when bleeding the brakes, there is a trick tool to "pin" that shuttle in the center. (I've made them using a
bench grinder and a 3/8-24 bolt)


ex-Global West GM
1991-1995

746459
 

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Try cracking open both brake lines just a little at the same time. Turn the key on and have someon just barely push on the brake pedal and it should center the switch and the light will go out. Whn the light goes out have them stop pushing and holde the pedal int hat spot. Do not let them release the brake pedal before you tighten the lines or air will be sucked back into the system. Wash any brake fluid off with water immediately or it will eat your paint. Better yet try and catch any fluid with a rag.
 

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Bleed all the brakes until the fluid looks clear. A good flush never hurts. Chances are good that the light will go off when you are finished. I wouldn't mess with the distribution valve unless absolutely necessary. You can plan on bleeding the brakes if you loosen any fittings on the valve. Equal pressure on either side of the piston is supposed to center it. In theory, stepping on the brake pedal should center the valve. If you flush the brakes and the pedal feels firm the piston may center itself if you push hard on the pedal.
 

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Just a note on the way the brake fail light works.
The distribution block, or Proportioning valve switch simply provides a ground for the light circuit. Yes just a ground.
67 has one post, 68-73 has 2 posts, yes both grounds. When shuttle slides one way or other, it contacts the spring loaded pin in bottom of switch, thus creating the ground. When pin is centered, it hangs free/clear of the schuttle, zero ground.
 
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