After replacing entire ABS unit and vacuum brake booster, brake pedal still goes to floor?

BRIANLCRAWFORD75

MEMBER
2000 GMC SIERRA
4.8L
V8
2WD
AUTOMATIC
207,000 MILES

I'm at the end of my rope. I'm a 25 year mechanic and I've done everything I can think of to my truck listed above 1500 so I've turned to the internet for help. The brakes were not working to par but were recently replaced so I knew that wasn't the issue. My first step was to replace the master cylinder, no dice, so I replaced the pressure modulator, again, no dice, so next was the ABS pump motor. Third time's a charm, nope. So I replaced the ABS control unit - you've got to be kidding, and lastly, the vacuum brake booster. All to no avail! I did all of the correct bleeding each time and there's definitely no air in the system but the brakes go to the floor when I turn the truck on (hold pressure when truck is off?) And I'm at my wit's end! Any suggestions other than buying a new truck? FYI, rear brakes are disc brakes and mileage is about 207,000. Any help would be fantastic! Thank you!

5 Replies

HARRY P

MECHANIC
2,292 POSTS

Knowing those trucks and the failures of aftermarket master cylinders, I'd go back to the master cylinder. It's not uncommon for an aftermarket MC to have a bad internal seal right off the bat, allowing fluid to blow by and not hold pressure like it should. I'd replace the MC again and change the brake fluid at the same time. A lot of time, spongy brakes are caused by aged hydraulic brake fluid being more compressible than it should be. I don't that's the some cause off your problem, but it would be a solid contributor at that age and mileage.

Just my 2 cents.

Harry

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Sunday, May 23rd, 2021 AT 8:07 AM

BRIANLCRAWFORD75

MEMBER
2 POSTS

Replaced the cylinder again but it's still aftermarket as I live in the middle of nowhere with no Chevrolet/GMC dealers. All fluid is new - replaced everything but still no dice. This is insane! Any more suggestions? Thank you!

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Tuesday, May 25th, 2021 AT 8:28 AM

JACOBANDNICKOLAS

MECHANIC
109,700 POSTS

Hi,

Did you perform an automated bleed on the ABS? Your description sounds like there is still air in it. That is where the issues usually occur.

I attached the directions below. Let me know if you have already done this.

Take care,

Joe

See pics below. Note the automated bleed actually starts at number 10.

Images (Click to make bigger)

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Thursday, May 27th, 2021 AT 8:46 PM

KEN L

MASTER CERTIFIED MECHANIC
47,513 POSTS

I would suspect there is air in the lines. These guides can help you fix it:

https://youtu.be/w7gUsj2us0U

and

https://youtu.be/WDxvEQrMkBg

and

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please run down these guides and report back.

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Saturday, May 29th, 2021 AT 2:38 PM

HARRY P

MECHANIC
2,292 POSTS

I'm with Ken. I've done some research and poked around AllData. If the problem is mechanical, it would have to be either the master cylinder or the brake pressure modulator. Since you've replaced both, I'm thinking the problem must be a stubborn air bubble somewhere. You may have to jack one part of the truck way up as you go around the vehicle bleeding at each wheel, but that bubble will get worked out. Go through the guides that Ken has referenced and let us know what you get. I think that'll get you straight.