Brakes Seem to Go Into ABS When Descending a Hill at Slow Speeds

[03msc]

So, the title pretty much explains the issue but I'll elaborate here.  2015 Crew Cab z71.  

 

I've noticed this in two different scenarios - one scenario, which was the first time I experienced it, is driving in the woods down a hill for a couple of hundred yards and, two, in stop and go slow moving traffic, also going down a hill/decline.  

 

The first scenario is going into a hunting cabin we have in the woods.  I first thought it was because it was through solid leaves and that maybe there was some slipping/sliding and it was trying to counteract that.  However, when it does it on dry pavement then that kind of throws that out the window in my mind, at least.  I was in stop and go traffic, crawling when we were moving. After about a mile of this, we were going downhill for about 100 yards or so and it did it - pedal feel complete changes, brakes make a grunting sound, and pedal slowly presses all the way to the floor.  Truck stops but certainly not normally.  The only way I've figured out how to get it to return to normal is put it in park, turn it off, and start it again. After that it'll be fine.  

 

Anybody experienced this?  Ideas?  What do you think?  Brakes otherwise operate fine and feel 'good' and not in need of new pads or anything. I'm about to roll over to 75,000 miles but it's been doing it, at least when going down the hill to the cabin, for 20,000 miles at least. (Don't go that often so it's one of those things that gets forgotten or isn't a high priority until it happens).  Had never done it in traffic on dry pavement until about 3 months ago though.  

 

Any insight or suggestions appreciated.  If someone else has experienced this and knows the fix I'm all ears.  

 

[JimCost2014]

1 hour ago, 03msc said:

So, the title pretty much explains the issue but I'll elaborate here.  2015 Crew Cab z71.  

 

I've noticed this in two different scenarios - one scenario, which was the first time I experienced it, is driving in the woods down a hill for a couple of hundred yards and, two, in stop and go slow moving traffic, also going down a hill/decline.  

 

The first scenario is going into a hunting cabin we have in the woods.  I first thought it was because it was through solid leaves and that maybe there was some slipping/sliding and it was trying to counteract that.  However, when it does it on dry pavement then that kind of throws that out the window in my mind, at least.  I was in stop and go traffic, crawling when we were moving. After about a mile of this, we were going downhill for about 100 yards or so and it did it - pedal feel complete changes, brakes make a grunting sound, and pedal slowly presses all the way to the floor.  Truck stops but certainly not normally.  The only way I've figured out how to get it to return to normal is put it in park, turn it off, and start it again. After that it'll be fine.  

 

Anybody experienced this?  Ideas?  What do you think?  Brakes otherwise operate fine and feel 'good' and not in need of new pads or anything. I'm about to roll over to 75,000 miles but it's been doing it, at least when going down the hill to the cabin, for 20,000 miles at least. (Don't go that often so it's one of those things that gets forgotten or isn't a high priority until it happens).  Had never done it in traffic on dry pavement until about 3 months ago though.  

 

Any insight or suggestions appreciated.  If someone else has experienced this and knows the fix I'm all ears.  

 

Two guesses, have you had the vacuum pump replaced or updated? Do you have the downhill decent on?

 

See if I can get this post rolling for you.

[03msc]

4 minutes ago, JimCost2014 said:

Two guesses, have you had the vacuum pump replaced or updated? Do you have the downhill decent on?

 

See if I can get this post rolling for you.

Not replaced and not updated, no. 
 

And no downhill decent on. Unless it’s default. But I think it requires a button press which I haven’t used. 

[64BAwagon]

What your describing is similar to what happened on the 99 through 2000's trucks when the front ABS sensors were "rust jacked" and would lose signal on the tone ring in the hub. There was a recall on them. Every full size GM truck we had at work needed to be repaired as well as mine since the dealer didnt do the recall proprerly. 

[MikeBMW]

Are you sure you're not getting a message that "hill descent control" is engaged?

[03msc]

1 hour ago, MikeBMW said:

Are you sure you're not getting a message that "hill descent control" is engaged?

I don’t see the message, no. 

[03msc]

Still curious if anyone else has experienced this.  It happened again yesterday and it's almost like a grunting sound as the pedal slowly continues to depress to the floor while braking.  Can feel it in the pedal, as well.  Yesterday it happened when I was reversing.  

[CamGTP]

A bad wheel bearing could do this. The ABS sensor/tone ring is built into the bearing so you replace the entire thing.

 

If you were able to catch it on a scan tool usually what happens is that a wheel will drop faster or drop to zero while the other wheels are still rolling, this will enable the ABS system.

 

The other thing is a weak vacuum pump. I every once in a great moon I will get it when reversing down the driveway. If I'm at a warm 550-600rpm idle and I've pressed the brakes once or twice before actually putting it into reverse the next time I hit the brakes the pedal will slowly go down and the ABS kicks in. Very seldom happens so I haven't replaced the pump yet.

[shahangsxr600]

Had the abs problem on my 2014 buick regal. It would come on and go off. I put a new wheel bearing on it and fix my problem. Good luck

[03msc]

Interesting insight on the wheel bearing or vacuum. Will keep those in mind. 

[03msc]

On 3/11/2023 at 1:48 PM, CamGTP said:

The other thing is a weak vacuum pump. I every once in a great moon I will get it when reversing down the driveway. If I'm at a warm 550-600rpm idle and I've pressed the brakes once or twice before actually putting it into reverse the next time I hit the brakes the pedal will slowly go down and the ABS kicks in. Very seldom happens so I haven't replaced the pump yet.

 

So when I read this the other day I thought "yeah I think that's happened to me" so I tested it a couple of times lately.  I can do what you described (press the pedal some before reversing down the drive) and it did it.  It also did it in a parking lot the other day as I had depressed the pedal a couple of times while sitting there before backing out to leave.  I'm thinking this may be it vs the bearing.  Maybe.  

 

What's involved in changing the vacuum pump?  I want to hope it's an easy thing but I'm going to guess it's probably difficult... 

 

What about the bearing?  Since both have been mentioned... 

[mikeyk101]

The vacuum pump should be a recall and the dealership should do it for nothing. There was a recall notice on it sometime in last couple years.

 

Enter your VIN # here and it should tell you if your vehicle has been recalled.

 

https://www.nhtsa.gov/recalls

Edited March 20, 2023 by mikeyk101

[03msc]

3 hours ago, mikeyk101 said:

The vacuum pump should be a recall and the dealership should do it for nothing. There was a recall notice on it sometime in last couple years.

 

Enter your VIN # here and it should tell you if your vehicle has been recalled.

 

https://www.nhtsa.gov/recalls

 

It only shows the "roof-rail airbag (“RRAB”) inflators" recall which I had done this morning and hasn't cleared the system yet I guess.  

[mikeyk101]

Have them check Special Coverage N182202780

 

https://www.carcomplaints.com/news/2022/gm-brake-vacuum-pump-recall-closes-investigation.shtml

[mikeyk101]

https://www.tsbsearch.com/Chevrolet/N182202780