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Dash Brake Light Still On !!

8

89Blazer4wd

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I had a rear brake hard line rust through and bust which caused the rear brakes to go out and the brake light on the dash to come on. Replaced the master cylinder thinking it could be that and then finally got into the rear brakes this weekend. Thankfully it wasn't the wheel cylinder and ended up just being the hard line to the pass. side rear wheel. I replaced that and bled the system out and the brakes are working great. Why the hell is the brake light on the dash still lit though? Where are the sensors that cause this to come on? Or how do i check whats causing it to come on?

Thanks,
Patrick
 
I know if your master cylinder is low the light will come on, did you double check the fluid level?
 
there is a differential pressure valve in the brake system. When a brake line blows out the valve inside shifts to the side that busted. When that happens it blocks off the brake line that blew so fluid can't be all lost out that way. There is a switch inside that when this happens the brake light is kicked on. Best way to fix it is, go out and slam your foot on the brake pedal a couple good stabs. Fixed it last time I had this happen. Not 100% an 89 has this setup but I'm pretty certain it does.
 
That might make sense. Because when the brake line first busted I kept checking the brake fluid in the resevior and it was always full. It wasn't until after I replace the master and then refilled it and bled it out that the rear brake resevoir wouldn't hold fluid anymore. I do know for sure that the fluid it full and i've bled out the whole system so i know fluid is making it to all 4 wheels. I'm thinking that after the pressure was taken off the system when i removed the MC that is free'd up the line. Just not sure how to get the stupid light to go off now, but i'll give your suggestion a try and repost later today.
Thanks.
 
If you want to fix it without getting teeth, hair and eyeballs all over your dash, just go under the truck and push the reset button on the combination valve.
It's on the back side of the front crossmember. There's a little rubber boot on one end of the valve, just push it in to reset the differential pressure spool.
 
Unplug the controll box on the side of the MC. Then plug it back in. I do mine every couple of months.
 
That won't reset the pressure differential warning switch.
 
goldwing2000 said:
That won't reset the pressure differential warning switch.
Click to expand...
No, but resetting the switch won't make the light go out either. On the later models, the box keeps the light on. Even if the problem is gone, the light will remain. I have seen this on 4 trucks, so I'm not sure if it is normal, or just a hiccup on the trucks I've seen.
 
Can you better describe the control box that you are referring to and where its located? I looked for it this morning before going into work and i found a box mounted to the rear fire wall to the rigth of the MC and unplugged two connectors that went into that, but i'm not sure what that even is. Either way it didn't turn the light off, when it started it back up it was still on.
also is there a better idea of where the reset button is under the truck? I haven't gotten under there yet, but not really sure what i'm looking for either.
Thanks, for the help.
 
Leper said:
Is there not a box mounted directly to the MC??? All of the 88-91 I have seen had it.
Click to expand...

No, there isn't anything electrical or wiring connected to the MC itself. Just the MC bolted to the booster. I can look again, but i'm pretty sure there isn't anything there.
 
Just an FYI but there is no box mounted to my MC on my 88 Jimmy.

Only help I can give you with the brake light is check your prop valve and reset it, also check your ebrake switch to ensure it is working correctly. I think they put ABS on some of the 90's trucks (I'm not sure on the years). If you have one of these systems I'm not too sure how to diagnose that.
 
Leper said:
No, but resetting the switch won't make the light go out either. On the later models, the box keeps the light on. Even if the problem is gone, the light will remain. I have seen this on 4 trucks, so I'm not sure if it is normal, or just a hiccup on the trucks I've seen.
Click to expand...
Leper said:
Is there not a box mounted directly to the MC??? All of the 88-91 I have seen had it.
Click to expand...

If you don't have ABS, you won't have a control module (box) for the brake system. If you do have ABS, you probably have to reset both the BCM (by unplugging it) and the pressure differential switch (by pushing the button).

The combination valve is located on the back side of the forward-most frame cross member. Just follow the brake lines down from the master cylinder.
 
isn't there a switch for the emer. brake pedal for the light also. Have you looked to see if the switch on that is bad?
 
Alright well I found the reset button under the truck. I pressed that a few times and that didn't do anything for me. I looked under the dash around the E-brake pedal and didn't see any connectors there. Where is the switch for that located?
It is weird that the light came on right when the brake line busted, and now its fixed and it's remaining on for something else. When i turn the key to the on position and the dash lights come on before starting. The brake light never goes out. stays on from the time you turn the key to on, through starting, while running, and until you turn the key off.
Does that info help with diagnosing the problem? That damn thing is on all the time, when's the bulb just going to burn out already!!??
 
It has to be the pressure differential warning switch. Did you feel the spool valve move when you pushed the end of the combo valve? Just poking at the rubber cap isn't enough. They are usually stuck pretty good and you have to put a good amount of pressure on it to reset it.

If all else fails, just do like sled_dog says and jump on the pedal a couple times. It should eventually work its way back over.

Just so you have a better understanding of how it all works, read this: http://auto.howstuffworks.com/master-brake2.htm

Worse comes to worse, you could always unplug the pressure differential switch. That's not the best idea, though.
 
Thanks for the info and the site. Very informative.
I did try pushing the reset button in a few times, but im not really sure how much pressure i used to do it. I guess i can give it another try and see how it goes. I'll post the outcome.
 
Another thing that worked for me, open one of the front bleeders (as if to bleed it), push the pedal to shift the spool in the combo valve the other way til the brake light goes out. Hold pedal there while helper closes bleeder. Don't let air in, unless you want to bleed the brakes again...hehe
 
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