69 mach - Can't get brake warning light to go off
#1
Hey guys,

I'm just about to call it quits when it comes to troubleshooting my brake warning light. I bought a proporting rebuild kit from cougarparts and finished the rebuild. I also changes the front brake hoses. The car brakes fine but I cant seem to get the brake light to go off. I even bought a brake light sensor tool which holds the piston centered in the distribution block while I bleed the brakes. That seemed to work initially until I started to drive and then the brake lights came back on. Any suggestions would be appreciated?

This is what I did so far: original front disc, rear drums

- new master cylinder
- new front brake hoses
- rebuilt proporting and distribution block
- brake sensor and wiring tested good
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#2
(12-23-2015, 09:29 AM)rindel Wrote: Hey guys,

I'm just about to call it quits when it comes to troubleshooting my brake warning light. I bought a proporting rebuild kit from cougarparts and finished the rebuild. I also changes the front brake hoses. The car brakes fine but I cant seem to get the brake light to go off. I even bought a brake light sensor tool which holds the piston centered in the distribution block while I bleed the brakes. That seemed to work initially until I started to drive and then the brake lights came back on. Any suggestions would be appreciated?

This is what I did so far: original front disc, rear drums

- new master cylinder
- new front brake hoses
- rebuilt proportioning and distribution block
- brake sensor and wiring tested good

Well the only thing that pushes the sensor out of position and turns on the light is a difference in pressure between the front and rear systems. Other then malfunctioning new parts(unlikely but possible) There is some air trapped, that has not been forced out. I don't remember if you tried any other way to bleed the brakes other then the conventional pumping, and cracking the bleeders. I have used a vacumn bleeder with a lot of success. and I remember others, using a pressure bleeder that forced fluid in from the top/ master cylinder, and out each individual bleeder furthest away to the nearest of course, and was very successful. I still say there has to be air in the system. One thing you may or may not of done. Is follow all the lines along the chassis and make sure you don't have any High or Sharp peaks in the way the lines are run. You could have one so high, that the trapped air can't be forced out during normal bleeding. Fluid doesn't move more then an inch or two with each bleeding stroke, and the air may move back to the Peak in the line before you can repeat the bleeding cycle to push the air out. Thus no matter how many times you bleed them you never get all the air out. Another thing I just thought about, The bleeder screws have to be at the top of calipers, I believe you can get them on the wrong side L or R and the bleeder will point down, and you would never get the air out of the top of the caliper. I hope this helps JTS 71 Mach1
"We built these cars to drive the "HELL" out of them, not to be museum pieces!"  Carroll Shelby

2008 Mustang V6 5 speed "Diablo Sport Predator" tuner, 87 octane tune. WOW!
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1986 Honda 450 Rebel
1995 Honda Pacific Coast
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#3
I agree, a vacuum bleeder or pressure bleeder is best for bleeding brakes. Vacuum bleeders seem easier and less messy than a pressure bleeder.

Is the rear brake hose also new? How does the pedal feel? Are the rear brakes adjusted correctly? Unrelated, but an emergency brake pedal that goes to the floor and doesn't work can be caused by improperly adjusted rear brakes. If all is okay, try a different bleeding method. There maybe trapped air in the system.

Regards,
Mike
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#4
Great discussion on bleeding of brake lines and personal preferences. This is why I like this site, always learning from the experience of others. Thank you for sharing!
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#5
+1 on the only thing that will trigger the warning light piston is unequal pressure in the two sections of the system.
Have you tried just pulling the wire off the switch and test if the light stays out? Does/did that piston move freely? Any debris or marks in the bore it moves in could affect the operation.
Is the switch defective in some way? If the pedal feels good and the car stops normally I would check those things.
                                   
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#6
Are the steel lines all new or relatively so? My pickup truck developed the same symptom. The problem turned out to be a rusted area on the steel line. The spot was underneath one of the clips that anchors the line to the frame. The rusted area couldn't be seen at first glance as the clip hid it. Nor was there a actual leak that was visible. The rusty spot was "damp" with brake fluid once the clip was removed but the fluid was seeping through and was not a full blown leak. Replaced the line and all was well. Dave R.
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#7
I havent replaced the metal lines. When I pull the plug out, the light goes out. Also when the piston is centered correctly the light goes out. Perhaps it is unbalanced pressure but I dont see any leaking and brakes feel fine.
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#8
Like others have mentioned, unbalanced pressure is the only thing that will cause the piston to move and trigger the light.

Regards,
Mike
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#9
(12-27-2015, 11:04 AM)rindel Wrote: I havent replaced the metal lines. When I pull the plug out, the light goes out. Also when the piston is centered correctly the light goes out. Perhaps it is unbalanced pressure but I dont see any leaking and brakes feel fine.

My truck acted the same way. The leak was so small that it was nearly undetectable. Check all of the lines under the clips that mount the lines to the body. The clips may be hiding very, very small rust perforations. Dave R.
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