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Old 01-05-2002, 09:49 AM   #1 (permalink)
diablopony
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Question Brake Light won't go out

I had a leaky rear drum brake cylinder that caused the brake warning light to come on. I replaced the cylinder and bled all the brakes, but the light won't turn off.

Any ideas on why bleeding didn't reset the pressure differential valve to center and how to fix this?
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Old 01-05-2002, 10:38 AM   #2 (permalink)
Kisner
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Have same problem with my 68 which I haven't got around to fixing since I'm not a mechanic. I do have original shop manual which states something to the affect that you have to remove either the inlet or outlet port on the differential valve opposite to the one bled last and pump the brakes to reset it to the center position. I can get the exact words and instructions from the shop manual if you'd like.
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Old 01-05-2002, 10:50 AM   #3 (permalink)
Kisner
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Went ahead and dug out shop manual. Here's exactly what it states:
"Centralizing the Pressure Differential Valve...to centralize and turn off warning light, a pressure differential or unbalance condition must be created in the opposite brake system from the one that was repaired and bled last. Turn ignition to ACC or ON position. Loosen differential valve assemply brake tube nut at the outlet port on the opposite side of the brake system that was repaired and/or bled last. Depress the brake pedal slowly to build line pressure until the pressure differential valve is moved to a centralized position and the brake warning light goes out; then, immediately tighten the outlet port tube nut to the specified torque. Check fluid level and fill to within 1/4 inch of the top. Turn ignition switch to OFF."

Now my problem is (not being a mechanic) I don't know which was bled last or exactly how to do this. Also, when I loosen brake tube nut, do I have to put a cup or something under it or will brake fluid come running out while I depress the brake pedal? Showing my complete ignorance again here.
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Old 01-05-2002, 12:24 PM   #4 (permalink)
Gearhead999
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Common problem. The valve has moved and has stuck. If you go down and look at the valve there is a electric wire going to the switch in the center. That is what turns the light on. Remove the wire and then unscrew the switch. You can then manually center the valve. Hold it in place, they use to make a little tool that would screw down in there to hold the valve while you bleed. You ought to be able to make something up. With the shuttle valve held in the middle. You bleed front and rear "softly". Don't go jumping up and down on the pedal or pushing like you're trying to check the floor for rust. When you get solid fluid front and rear, screw the switch back in and just step on pedal lightly. It should be OK.

What ever don't just disconnect wire to put light out. You will have only half of your braking system, cause the valve has moved off center.
Hope that helps.
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Old 01-05-2002, 01:46 PM   #5 (permalink)
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FWIW, the manual transaxle neutral adjustment nut and pin from an early 80's K-car is an exact match for the centering pin mentioned above.

Take care,
-Chris
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Old 01-06-2002, 03:14 PM   #6 (permalink)
diablopony
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Question

Man this one is a real pain! The valve is bolted to the fenderwell right under the master cylinder. I was able to disconect the wire to the pressure differential valve and remove the nut on top, but I can't really get to anything from there.

Short of pulling the master cylinder, any other thoughts on this?
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Old 01-06-2002, 03:29 PM   #7 (permalink)
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Unfortunatly I don't think there is much else you can do. Have you tried going from underneath??

I had so much trouble with mine. Couldn't keep the rear wheels from locking on any type of sudden stop. I removed it when I put the SS rear disc brakes on my 68. I installed an inline manually adjustable valve on the rear brakes. Took it on the road and did some braking and some adjusting. Took several stops and adjustments to get a happy median, but works great. Oh, yeah stops good too.
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Old 01-06-2002, 08:18 PM   #8 (permalink)
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My 68 doesn't resemble this setup. I know it's all original, but I have a "late" model 68 where I know several minor changes were made, i.e., deleting the pillar pads, knee pad, using the Cougar side reflectors, etc. My brake warning switch is a plug in. Is there any way of removing it and just sliding the rod back to a center position with a screw driver or something without tearing something up?
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Old 01-06-2002, 08:49 PM   #9 (permalink)
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Quote:
Originally posted by Kisner
My 68 doesn't resemble this setup. I know it's all original, but I have a "late" model 68 where I know several minor changes were made, i.e., deleting the pillar pads, knee pad, using the Cougar side reflectors, etc. My brake warning switch is a plug in. Is there any way of removing it and just sliding the rod back to a center position with a screw driver or something without tearing something up?
Unless I'm missing something here, yours sounds exactly the same. The details outlined by Gearhead are perfect. Just do what he said if you ever have this same problem. In effect, he already answered your question.

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Old 01-06-2002, 09:38 PM   #10 (permalink)
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Kisner,

My car is also a late 68 with all the same parts you are talking about.

As I said if you take the switch out of the valve you can manually center the valve. Sorry, I didn't say it was easy. So if you have to pull the mastercylinder off, you have to pull the mastercylinder off. Two lines and two bolts off it comes. Then you can see the valve clearly. I would use some sort of pick [ice or dentist] to move it. Remember patients is how to get it done.

I don't mean to offend, but if you don't understand the brake system and aren't comfortable working on this, then by all means don't. You shouldn't play around with brakes, you never know when you might need them !!! Find yourself a good garage that will do this type of work. I own a garage here in Orlando, FL and we work on many older cars. In the last 30 days....1960 Corvair, 1960 T Bird, 1961 T Bird, 1948 Chrysler, 1966 Mustangs [2], 1977 Triumph Spitfire, etc. There has to be someone like my shop near you.

These valve stick cause of the age and dirt gets in there and they get sticky. Thats why I took it off my car.

Good luck.
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Last edited by Gearhead999; 01-06-2002 at 09:58 PM..
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Old 01-07-2002, 12:29 PM   #11 (permalink)
Kisner
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I was wrong (bad memory chip in my brain) - it is the same setup as yours. The wire has clips on it that push on the switch. The switch itself screws in. I'll give it one shot if I can get lucky without removing master cylinder. If not, I definitely am not a risk taker when it comes to something I don't have experience with and I'll take to my mechanic.
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Old 01-07-2002, 07:08 PM   #12 (permalink)
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I'm OK with pulling the master cylinder. I've done so several times. It is just something I rather not do if I can avoid it because, as you say, brakes are some things that you want to be sure of. Therefore, you need to spend time checking and double checking to be sure all the air is out and everything is properly connected.

That is why is use this forum. Many times someone will have a suggestion on how to approach the problem different from the Chiltons manual and without disconnecting everything.

Looks like I struck out on this one, but maybe next time.
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Old 01-09-2002, 06:31 PM   #13 (permalink)
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Unless your master cylinder is leaking uot the rear (in which case it needs to be replaced anyway), you run no risk unbolting it to move it out of the way. Just don't loosen the line nuts.

Take care,
-Chris
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