Dash brake light keeps coming on

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bigcountry78

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After we did the throwout bearing, I hooked the parking brake cable back up. I did the reverse technique to adjust the rear brakes because the parking brake wasn’t holding very strong, and then I drove it 100 miles back home. Now when I push the brake pedal moderately hard, like for a typical stop at a stop sign, the brake light on the dash comes on and stays on until I tap the brake pedal once or twice. Then it comes back on at the next stop. Fluid reservoir is full. Any ideas on what to check next? I didn’t have any problem until we worked on it this weekend.
 

chengny

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After we did the throwout bearing, I hooked the parking brake cable back up. I did the reverse technique to adjust the rear brakes because the parking brake wasn’t holding very strong, and then I drove it 100 miles back home. Now when I push the brake pedal moderately hard, like for a typical stop at a stop sign, the brake light on the dash comes on and stays on until I tap the brake pedal once or twice. Then it comes back on at the next stop. Fluid reservoir is full. Any ideas on what to check next? I didn’t have any problem until we worked on it this weekend.


I'm not too familiar with the earlier model trucks. Most of what I know is just what I learn from the service manuals/wiring diagrams - so consider the source when reading the following.


From what I can see (in model year 1978), it doesn't look like the e-brake had a switch that could cause the e-brake warning light to come on. Up till that year, the warning light could only be illuminated by the switch in the BPC valve. It looks like 1979 was the first year that an "e-brake applied" switch was incorporated into the circuit.

Should be easy enough to check whether I'm "shootin' it out my ass". Just get under the dash and look up at the e-brake engaging mechanism. If you see a switch with a wire (should be TAN if it exists), then I'm wrong. If you don't see a switch/wire, the problem is with the BPC valve.

From what I read above, the job you recently did (changing the throwout bearing) shouldn't have had any effect on the BPC valve - or any part of the hydraulic braking system. Also, if - and that's a big if - there is no e-brake warning switch, unhooking/rehooking the parking brake cable couldn't cause the warning light to start coming on.

Take the BPC valve switch out of the equation by pulling the TAN/WHT lead off the switch terminal. If that stops the warning light from coming on, there is something going on with the BPC valve.
 

PrairieDrifter

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I know in a lot of well used trucks that the parking brake lever will get slopped out and sag which would turn the brake indicator light on. Seen many many trucks with the park brake pedal tied up with wire or twine to keep it from sagging and tripping the light
 

75gmck25

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There is a brake proportioning valve mounted up under the radiator support on the driver's side, and it has a wire connection for that brake light (light colored wire, maybe tan). If you push on the pedal and the flapper piece inside the valve goes too far in either direction (uneven pressure front to back), it turns that warning light on. It usually re-centers itself in a short time and the light goes off.

I had the same problem on my '75 and it turned out to be a leaking wheel cylinder on one rear wheel. It leaked enough to drop the rear brake pressure and activate that switch. I was happy to get the warning; however, with a 14 bolt FF it was not a simple brake job. On a FF you have to pull the axles to get the drums off, and then I had to buy new 13" brake drums. Not a cheap brake job. It will be a lot easier to fix on your 1/2 ton. Or maybe you just have a brake line leaking.

Bruce
 

bigcountry78

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Mine doesn’t have the switch. If it does, it has never worked. Pretty sure it isn’t there. Sounds like it’s the valve. Are replacements available?
 

78C10BigTen

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There is a brake proportioning valve mounted up under the radiator support on the driver's side, and it has a wire connection for that brake light (light colored wire, maybe tan). If you push on the pedal and the flapper piece inside the valve goes too far in either direction (uneven pressure front to back), it turns that warning light on. It usually re-centers itself in a short time and the light goes off.

I had the same problem on my '75 and it turned out to be a leaking wheel cylinder on one rear wheel. It leaked enough to drop the rear brake pressure and activate that switch. I was happy to get the warning; however, with a 14 bolt FF it was not a simple brake job. On a FF you have to pull the axles to get the drums off, and then I had to buy new 13" brake drums. Not a cheap brake job. It will be a lot easier to fix on your 1/2 ton. Or maybe you just have a brake line leaking.

Bruce
My 78 c10 currently has that same problem but i never have time to fix the cyl.
 

75gmck25

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Maybe your proportioning valve is a little too sensitive, and replacement valves are available from specialty parts sources or from the junkyard. However, what you probably need is a rear brake job. Something is allowing a pressure difference in the system, and that is tripping the valve.

You may not realize the loss in braking power, since 1/2 your system is still probably functioning normally.

Bruce
 

bigcountry78

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Maybe your proportioning valve is a little too sensitive, and replacement valves are available from specialty parts sources or from the junkyard. However, what you probably need is a rear brake job. Something is allowing a pressure difference in the system, and that is tripping the valve.

You may not realize the loss in braking power, since 1/2 your system is still probably functioning normally.

Bruce
I think you may be on to something. I’ve noticed the last couple weeks that occasionally when I’m sitting at a light on a slight incline, when I let off the brake the truck doesn’t roll forward. It’s like the brakes are still on. And yesterday at a light, I felt a clunk in the rear end like something was hung up and then let go. I’m going to have to pull the rear drums off and see what is going on.
 

RoryH19

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Had the same problem on my 82 K5. Not surprised since the truck sat for 14 years.
I think leaking wheel cyl started the problem but it was also a bad proportional valve. It was frozen in the position to keep fluid from flowing to the rear drums. I also changed the master cyl to be on the safe side.
Works fine now.
One thing to note is that once you replace the wheel cyl you may need to bleed the front brakes first to relieve the pressure. Then you will be able to bleed the rear.
 

rt66paul

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You could have a bad spring on one side and have a shoe dragging, or even tightening up the e brake was enough to break something in there. If it has been sitting for years, rust can do many bad things. Save your pennies and consider all the brake hardware along with the rear brake job.
 

RoryH19

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You could have a bad spring on one side and have a shoe dragging, or even tightening up the e brake was enough to break something in there. If it has been sitting for years, rust can do many bad things. Save your pennies and consider all the brake hardware along with the rear brake job.

Right, I changed all the drum hw to be safe. Blew out all the steel lines and replaced the rubber ones. All works as it should. The proportional value frozen. I couldn't even remove the sensor from the value it was so rusted on.
Was better to start with new parts. Brakes are a must.
 

bigcountry78

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Well I don’t know what happened, but the light has stopped coming on. My brakes have started squeaking more though. I know the front brake pads are in good shape, I looked at them when we had the tires off to paint. I need to take a look at the rears though.
 

bigcountry78

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Looks like I get to do the rear brakes sooner than later. Came home to find this tonight

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