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i have a 1970 nova ss with power brakes disc/drum and recently replaced the master cylinder (bench bled ) and bleed all the brakes and can not get rid of the idiot brake light? i pulled the pressure diff. switch and noticed a brass rod inside and did not notice anythig else in there, is this correct?? before removing the switch, i would put my foot on the brake pedal and the rears would lock up and would not get any in the front. this prompted my removal of the switch to see about resetting.
 

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Sounds like you are going to have to take the thing completely apart and clean out all the gunk in it in order to get the pressure differential slider to move freely.
 

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it needs replaced. GM's are self centering, if it was a Ford, you unscrew the contact, and move the inside plunger over with a pin. if it's a GM and it's not self centering, it needs replaced.
 

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Is there a little pin sticking out of the front of the valve? Or a rubber thing with the pin hidden underneath it? Rig up a way to hold that pin inwards while you bleed the system again. That should fix it. If not, the valve may be bad. I've heard they're hard to find. If you're absolutely sure the brakes are working properly, bulb deletion may be a viable option.
 

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Loosen one of the bleeder screws, front or back and gently press the break pedal. If the switch is displaced to the front breaks meaning that front breaks were faulty, loosen the rear bleeder screw after the front breaks are all full of fluid and repaired if leaking and that should put that switch right back where it belongs when you push the break pedal. Sometimes you have to be a little more persuasive with that pedal but you get the idea. If that doesn't work try loosening the fitting right at the switch. It There must be no leaks on your system any where else. This has not ever failed me yet.

After you get the light to go out bleed the side that you opened and your all set.
 
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