First, I apologize if this issue has been addressed already. I've looked a few times and haven't found my issue.

My Suburban brakes automatically gradually apply themselves. All four at the same time. It has done it a handful of times in the past year randomly. When it happens, I only notice it because it takes more gas pedal than normal to maintain speed. Then, it gradually gets to the point I have to floor it just to keep the vehicle moving at all. I've changed the master cylinder (Delco), hydroboost has been changed once by me then again with another one by a mechanic, new pads rotors and calipers in the last year. Sometimes when this happens, I'll get a service stability system notification on the display. Thought I fixed the problem by changing the master cylinder a few months ago--hasn't done it since...until today. Today I was pulling a empty 3 horse trailer UP a mountain pass and i got a service stability displayed. Gradually, i noticed the vehicle struggling to maintain speed. Eventually it was so bad I was in 1st gear trying to maintain 20mph going UP hill (no brakes applied). I pulled over and all four wheels were literally smoking. I cracked the lines on the master cylinder and it relieved pressure and unlocked the brakes. I also pulled the ABS fuse (thinking in the past this resolved the locking brakes issue). Still did it with ABS fuse pulled. I don't know where to go from here. When I had it at the mechanic in January 2020, he said he pulled up C0035, C0040,C0110,C0263, C0131, C0455 and cleared them.

 

Have you ever gotten an abs warning?

 

Never. Only a "service stability" notification on display. I updated original post to display the trouble codes the mechanic pulled way back in January.

 

Make sure that there's the correct amount of brake pushrod free play.

 

Could be the abs pump

 

Haven't checked it, but I can't imagine that being the issue. I've replaced hydroboost twice in the past year. Besides, the brakes only lock up very rarely, but normally work...normally. This is a gradual complete lockup of all 4 brakes with zero foot on pedal. How would a incorrect length pushrod cause that issue?

 

That's what I'm exploring now. ABS pump, brake pressure modulator valve assembly. I guess all that stuff is co-located? I'm going to check wiring harness first. Not sure how I'm going to duplicate the problem. It might be 4 months before it happens again.

 

Hey man did you ever find the issue?? I’m having the same problem with my 05 Tahoe

 

Thanks for commenting--I totally forgot to update!

So the solution was super simple. There's a bracket that was placed between the hydroboost and master cylinder. That MUST be in place (or equivalent)

for brakes to work properly. When I last replaced my master cylinder, I remember thinking "that's a strange place for that bracket to go, it probably goes on the outside (forward side) right under the nuts that hold master cylinder to hydroboost". Well, putting the bracket on the outside was wrong. It allowed the master cylinder to sit closer to/right up against the hydroboost. That little distance caused the brakes to be always "a little bit" applied, it increased pressure, which then eventually locked up brakes. So I kept the bracket on the outside, but added a washer that was the same thickness of bracket ;and placed it in between master cylinder and hydroboost. Never had a problem since. Perfect example of Occam's Razor in play: the simplest solution is usually the right one.

 

I read your fix, great !!!

I wonder if adding the bracket essentially provides a correct amount of free play.....