Stumped by ABS

[Tahoeoh]

Hello. I have an 08 Tahoe LTZ 5.3, with ~ 215k miles, all of which are mine. I recently replaced the vac booster because it was hissing and the brakes were beginning to fail. I noticed a lot of brake fluid inside when I pulled the master cylinder off, so I replaced it as well. Now, the brakes pump up fine and release fine until the engine is started. Once started the pedal is spongy and very slow to return. Until the vacuum is released, that continues. The same result occurs if I release the pressure from any of the brake lines.I noticed that when the pedal is making its slow return, if I push the accelerator, the brake pedal stops until I release the throttle.

I have performed two gallons worth of bleeds, both manual and automated, using a Autel scanner. I have replaced the pressure sensor; replaced the brake switch; smoke tested the booster… found no leaks; checked the caliper slides… they seem fine; verified .02 inches between the booster shaft and master cylinder; and checked all connections, fuses and relays.

Even with the ABS and TCS lights on, there are usually no fault codes, The one I sometimes get is C0161 code indicating the brake switch circuit, but as mentioned above, the switch is now new. When the code pops up, it invariably shows “passed this ignition.” Live data shows 0 commanded voltage at the EBCM, but also says the ABS and Traction Control have failed, so I’m thinking there’s a relationship there. All wheel speed sensors are working and all signals read “valid” on the scanner.

Since this started with replacing the booster and master cylinder, I can’t help thinking one of them was bad when purchased. I have already bought a second master cylinder to no avail. Now, I decided to see if anyone on here has any ideas, before I take a chance and drop another $300 on a new booster. If it’s bad, I can return it for a refund, but if it’s not, that’s honestly on me…

 

[ReaperHWK]

Did you do the ABS bleed procedure with the scanner?

 

[rdezs]

It's been awhile since I had to deal with a vacuum booster..... What comes to mind is the one way vacuum check valve. On some vehicles it's in the vacuum line, and on some it's built into the end of the vacuum hose where it plugs into the booster.

1. Any chance you have that backwards? Or you are missing it? If it's not there, you'll lose all vacuum as soon as you turn the engine off. That results in a hard pedal. The amount of vacuum power assist would vary with manifold vacuum in that case.

2. Did you follow the procedure to bench bleed the master cylinder prior to installing?

3. Vacuum booster is pretty easy to test. Start the engine so that it gets a vacuum. Then turn off the engine. Now unplug the hose from the vacuum booster and you should hear the air getting sucked in. If not it's either not holding a vacuum and effective, or you are missing your check valve. If it was holding a vacuum, it's just fine.

Alternately you can start the engine for a minute and shut it off. Give it 5 minutes. Then feel the brake pedal as you apply it. You should have enough vacuum to give you power brakes for one or two applications and then the pedal should get hard. If it's hard right away, you're either losing your vacuum from a defective vacuum booster, or more likely your check valve is missing or bad.

 

[Geotrash]

Yes, he bled them with the scanner.

Something is wrong with the booster or the installation would be my first thought. Triple check the installation, pushrod end play and side rubbing etc. If it all checks out, I would order a combined booster and master cylinder from a reputable company such as Rock Auto.