Brake light Illuminated

Tiny
CASHFLOW50$
  • MEMBER
  • 2003 GMC YUKON
  • 4.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 180,000 MILES
My SUV is the SLE model.

Just replaced my brake pads and rotors on all four wheels yesterday. Bled the system with the vacuum connected to compressor and took the truck for a spin (with no warning lights on DIC). Brake pedal too low and spongy, so I knew there were still bubbles in the system. Instead of connecting the vacuum, I bled the system the two man old fashion way (pump pump pump and hold on the brake pedal). Started the bad boy up and the 'brake light' and 'service brake system' is illuminated on the DIC. Took it for a spin and had full stopping power. Brake engaged at minimal pedal press and was not spongy. The thing is what could have tripped the brake light and service brake system on the DIC? My partner and I was baffled and tried googling if anyone knew how to reset or fix this. We spent about two hours trying to recreate the steps that we could have gone wrong, but everything looked good. Does anyone have a clue. Need help cause this is my wife's ride and will not use it unless I clear the codes.



prior fixes but might be unrelated:

replaced ECBM in 2013 (ECBM repaired by Module Master).





please help. Thanks.
Do you
have the same problem?
Yes
No
Wednesday, April 26th, 2017 AT 4:48 PM

11 Replies

Tiny
CARADIODOC
  • EXPERT
Why did you need to bleed the system? Were parts replaced that let air in or brake fluid out?

When you pedal-bled the system, at any time did the pedal get run more than half way to the floor? That must be avoided on any vehicle more than about a year old. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons do not normally travel. Running the brake pedal over half way runs the rubber lip seals over that crud and can rip them. The typical symptom is a slowly-sinking brake pedal, and that often does not show up until two or three days later. We will assume for now the master cylinder is okay since the brake pedal seems good now.

To fit the new, thicker pads over the rotors, you needed to pry the pistons into the caliper housings. Once everything was assembled, it is necessary to pump the brake pedal repeatedly to run those pistons back out to adjust them. One piston will always get there first, then, as you pump the pedal again, pressure starts to build in that half of the hydraulic system. No pressure has built up yet in the other half because one or both of those two pistons still need to travel further. The difference in pressures trips the pressure-differential valve, and that turns on that switch and the red "Brake" warning light.

The anti-lock brake computer monitors a lot of things and will set a diagnostic fault code when it detects a problem. It turns the system off at the same time, leaving you with the normal base brake system, and it turns on the yellow "ABS" light to tell you. The red warning will also turn on if the parking brake isn't fully released, and on most vehicles, if the brake fluid level in the reservoir is low.

The ABS computer sees the red warning light is on, but it does not know why. It only knows the reason could affect its ability to operate the system properly in an emergency situation. The pressure-differential valve could be tripped due to a leak in half of the system, and that certainly would prevent it from operating those two wheels. The bottom line is start with the red warning light, then read and erase the ABS fault codes. If the fluid level is low, handle that. If it is okay, the pressure-differential valve might be sticking. Chrysler and GM valves are spring-loaded to center themselves, but if they stick, a solid, hard jab on the brake pedal will usually free it. You may also need to open a bleeder screw, have a helper slowly push the brake pedal until the warning light goes off, have them stop right there, close the bleeder screw, then allow them to release the pedal.
Was this
answer
helpful?
Yes
No
+1
Wednesday, April 26th, 2017 AT 7:49 PM
Tiny
CASHFLOW50$
  • MEMBER
To answer your question about why we bled the system: My partner thought it would be nice to change the fluid as well, since it was dirty (greenish) instead of clean (light yellowish).

To answer your question about when we pedal bled the system: Yes it went down more than half. Sometimes all the way to the floor on some wheels. I cannot recall which wheel had more run. Also to add to that, I have changed the master cylinder prior so it is good without any problems. Pedal seems good about a 1/4 pedal press and it engages well.

I will see my nephew who works in a shop that has a tech I or Tech II. I will tell him to read the codes and then erase them. Then go from there.

You mentioned open up a bleeder screw? Are those the ones by each wheel or you talking about by the master cylinder? If it is the ones by each wheel, which one would be preferred? Thanks again for the response. I hope you can help me resolve this mess.
Was this
answer
helpful?
Yes
No
Thursday, April 27th, 2017 AT 9:13 AM
Tiny
CASHFLOW50$
  • MEMBER
Oh forgot to ask. So where is this pressure-differential valve located? Or which component houses it?
Was this
answer
helpful?
Yes
No
Thursday, April 27th, 2017 AT 9:18 AM
Tiny
CARADIODOC
  • EXPERT
Brake fluid likes to absorb moisture from the air. Besides leading to corrosion of metal parts, that lowers the boiling point considerably, leading to one type of brake fade. All manufacturers recommend periodic fluid replacement to get that moisture out, but very few of us actually do it. The reason I asked this is it is likely the reservoir did not run empty, as it might if there was a leak. If air goes down the lines, a lot of vehicles with anti-lock brakes must be bled with a scanner. It is needed to tell the computer to open some valves so trapped air can be expelled from some of the chambers.

Normally the pressure-differential switch lives in the combination valve. That is a brass block mounted on the frame, right below the master cylinder. You can follow the two steel lines from the master cylinder to it. That can change when anti-lock brakes is added, but there still needs to be a way to warn the driver when one part of the hydraulic system isn't working. Many GM vehicles, particularly front-wheel-drive cars, have a valve in the master cylinder that moves to block the two ports for the circuit that has a leak. This results in you being unable to notice a low or unusual pedal feel or travel. The only way to know there's a problem is by the red warning light, and that is turned on by the pressure-differential switch, as well as the parking brake pedal, and when used, the low-fluid level switch in the reservoir.

On Ford vehicles that pressure differential switch is not spring-loaded, and is REAL frustrating to reset after a repair is completed. Suppose the hydraulic system is split front and rear, and the rear system springs a leak. The pressure in the front system pushes the valve off-center which turns on the warning light. To reset the valve, you must open a front bleeder screw so that half of the system can't build any pressure. Now a helper slowly pushes the pedal until the valve centers itself, and the light turns off. At that point he must stop instantly and holler for you to close the bleeder screw, THEN he can release the pedal. The problem is that valve can stick, then suddenly pop the other way and turn on again. That happens so fast, you don't see the light go off for that brief few milliseconds. Eventually you can't be sure which way the valve has moved to or stuck at.

GM and Chrysler valves ARE spring-loaded, but I have had them stick on my vehicles. In every case a quick jab to the brake pedal snapped them free, then they centered themselves, and the warning lights turned off. In the rare event that doesn't work, you have to resort to the Ford method, but it''s not as frustrating.

I'm heading out of town for the weekend to an old car show swap meet. Will be back Monday to see how you're doing.
Was this
answer
helpful?
Yes
No
+1
Thursday, April 27th, 2017 AT 1:58 PM
Tiny
CASHFLOW50$
  • MEMBER
Thank you for the reply. I'll try the jab technique when I get home (with the engine ON or OFF?). So since my vehicle is a GM, I don't have to resort to the Ford way if I'm getting what you're saying. I've read in another post in here that I can test if its the parking brake that's causing the code. It said with the engine off and the key in the 'ON' position, press the parking brake and check the brake light on the dash to see if it brightens. Well I tried that yesterday and it didn't seem to change brightness. Does that mean my Parking Brake switch could be faulty, since it didn't change brightness?
Was this
answer
helpful?
Yes
No
Thursday, April 27th, 2017 AT 2:41 PM
Tiny
KEN
  • ADMIN
Hello,

If I may but in here, to me it sounds like you have a master cylinder that has gone bad which can be triggered by doing the brake job. The light is on becasue there is an imbalance in the brake system.

Here is a guide that will help you get the problem fixed.

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Please let us know what happens.

Cheers, Ken

Was this
answer
helpful?
Yes
No
+1
Saturday, April 29th, 2017 AT 3:51 PM
Tiny
CARADIODOC
  • EXPERT
Yup, the master cylinder is a good possibility if the brake pedal was pushed over half way to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Running the pedal over half way runs the rubber lip seals over that crud and can rip them.

As for the parking brake light trick, the change in brightness is REAL subtle, and anyone less intelligent than me would likely not notice it. There are three switches that can turn on the red "Brake" warning light. Once any one of them has turned that light on, either second switch provides an additional current path, and that very slightly lower total resistance will make the light just a fuzz brighter. All you're looking for is that very tiny change in brightness to show that something is happening when you push the parking brake pedal. That simply means the warning light was not on before due to the parking brake pedal.
Was this
answer
helpful?
Yes
No
+1
Monday, May 1st, 2017 AT 10:27 PM
Tiny
CASHFLOW50$
  • MEMBER
The proportioning combo valve is the culprit. I pulled the plug out and sure enough the light went off. I'll do the Jab thing first. If that doesn't work then i'll try and open up a bleeder screw and push on the pedal till the light goes off. If that doesnt do it. Then I'll follow the link that Ken provided, which is more thorough. If that doesnt work then I guess i'll have to replace the darn thing, but that requires me to bleed the entire system again.
Was this
answer
helpful?
Yes
No
Wednesday, May 3rd, 2017 AT 12:50 PM
Tiny
CARADIODOC
  • EXPERT
Holler before you replace the valve assembly. There are ways to avoid having to bleed at all the wheels.

I've never had one yet that we couldn't get reset with not much effort.
Was this
answer
helpful?
Yes
No
+1
Wednesday, May 3rd, 2017 AT 1:08 PM
Tiny
CASHFLOW50$
  • MEMBER
Hey CARADIODOC.
Update. Still haven't plug the sensor back in to this point. Brakes lots of stopping power. My partner and I will be doing the fix you instructed soon. I'll update you if it works or not. I don't want to spend anymore $$, so trying to fix this issue is a must. Thanks for your very informative fixes that I have yet to try. I'll be back.
Was this
answer
helpful?
Yes
No
Friday, May 26th, 2017 AT 5:04 PM
Tiny
KEN
  • ADMIN
Please let us know what happens.

Cheers, Ken
Was this
answer
helpful?
Yes
No
Saturday, May 27th, 2017 AT 11:55 AM

Please login or register to post a reply.

Recommended Guides