Help with brakes before I pull my hair out

[cofferson]

TLDR: I was having brakes sticking issues. After replacing everything I can't get a firm pedal. Wondering if ABS valve failure is common.

I own a 2008 Suburban 2500. The brakes were dragging so I started going around all four corners replacing pads and rotors. I also replaced the rear calipers (fronts are newer) and and guide pins and caliper brackets as necessary. Greased everything up and bled the brakes easy enough. Went for a drive and all felt fine but quickly turned sour. Brakes started dragging and then after hitting the brakes particularly hard, they stayed fully engaged. I punched the brake pedal, heard a light pop and the brakes released. At this point I changed the soft lines thinking MAYBE those were bad. They were perfect all the way through. I then figured that since it sat for a long while, it may be that the master cylinder piston is sticking. Replaced both the master cylinder AND the hydrobooster because the hydrobooster was leaking. Thankfully I did this because they were seized together and still are. I bench bled the master successfully and put the new parts on and went to bleed the brakes. Never really got any air out. Now I have an extremely soft pedal. Pull the lines back off and air got back in the master. Bench bled it again and put on vehicle with plugs and got a very firm pedal. Put the lines on and this time I didn't use a vacuum bleeder incase I somehow sucked in air, still a very soft pedal. So now I used a scan tool to bleed the ABS and still never got air out. At this point I'm at a loss because brakes are very simply. I feel like I'm chasing a ghost and at this point the only thing I can think is the ABS module has something wrong with it and once I replaced the master cylinder I basically screwed myself. Truthfully I don't know exactly how the valving works to understand the root cause myself so I'm wondering if anyone here has thoughts on if this could be the abs unit? It may even be that the ABS is what was causing issues all along and now it's either not allowing air through due to valves sticking or it's bypassing the pressure.

Thanks guys. I really want to get this on the road and it's making me highly regret even buying it even if that's an irrational thought..

 

[justchecking]

Have you checked again to make sure you have the new calipers on the correct sides?

 

[cofferson]

Have you checked again to make sure you have the new calipers on the correct sides?

Yes

 

[justchecking]

Man. On my 2011 I accidentally let all the brake fluid drain out while I was running to the auto parts store to get parts. I was able to bleed end to end with no problem. Are you pushing the pedal all the way to the floor or only 3/4 of the way? Should only be 3/4. Other than that I don’t know. Good luck!

 

[Rocket Man]

I highly recommend the Motive power bleeder especially in cases like this. It uses pressure instead of vacuum so there’s no way to introduce air into the system unless you let it run dry. It also makes it super easy for one person to bled the entire system. Also you don’t need a compressor running to create a vacuum like the ones that (literally) suck. Make sure you get the one for late model GM vehicles.

 

[Charlie207]

I only have a 1500 chassis, but whenever I do brake work it takes a couple days for the pedal firmness to fully return. This includes not even cracking any brake lines/draining fluid, but just compressing the calipers to make it easier to remove and reinstall after doing other suspension stuff.

Try parking on a steep uphill slant for a few days to let all the potential air bubbles gravity bleed.

 

[cofferson]

Man. On my 2011 I accidentally let all the brake fluid drain out while I was running to the auto parts store to get parts. I was able to bleed end to end with no problem. Are you pushing the pedal all the way to the floor or only 3/4 of the way? Should only be 3/4. Other than that I don’t know. Good luck!

Thanks. This kind of affirms that something is wrong. I've replaced master cylinders and have redone entire brake systems many of times but never on ABS vehicles. I have this unfortunate hunch that the abs valving is leaking internally causing either trapped air or bypassing the pressure. Because I've done everything I can think of and it's a very soft pedal.

 

[cofferson]

I highly recommend the Motive power bleeder especially in cases like this. It uses pressure instead of vacuum so there’s no way to introduce air into the system unless you let it run dry. It also makes it super easy for one person to bled the entire system. Also you don’t need a compressor running to create a vacuum like the ones that (literally) suck. Make sure you get the one for late model GM vehicles.

Thanks. I used a pressure bleeder as well to no avail.

 

[Fless]

Check out Eric O's procedure and tips.

 

[swathdiver]

TLDR: I was having brakes sticking issues. After replacing everything I can't get a firm pedal. Wondering if ABS valve failure is common.

If the ABS module or pump was no good, you'd get codes for it.

Did you follow this exactly? How many quarts of fluid did you go through? I use 4 to 6 each time.

 

[j91z28d1]

what issue are you trying to fix. mushy brakes that need bleed or brakes rust won't release?

 

[petethepug]

Brake dragging can be deteriorating brake lines. They deform and allow pressure to be retained in the lines.

They should be upgraded to braided stainless to prevent expansion as well as swapped out every decade.

 

[rdezs]

Brake dragging can be deteriorating brake lines. They deform and allow pressure to be retained in the lines.

They should be upgraded to braided stainless to prevent expansion as well as swapped out every decade.

Pete is spot on with this. The OEM brake lines are an issue, breaking down on the inside.

I also use the Motive pressure bleeder... Never an issue with it. And always follow up by commanding and abs bleed with an aftermarket by directional scanner.

If all that has been done, I would be looking at that master cylinder. First verify the part number is correct for your application. If so, return it as defective and pick up a genuine GM or AC Delco. (New, not rebuilt)

As you said, brakes are pretty simple. You have a defect somewhere and there's only so many somewheres that exist.

 

[j91z28d1]

from what I read he's replaced the rubber lines and ran abs bleed with a scanner.


being a hydro boost, it seems we just had someone here post about replacing a master and the push rod length being to long. double check that. if your rod between the 2 is slightly to long and it doesn't let the piston in he master completely release. it will cause all kinds of issues.

double check that. needs a bit of a air gap when fully releasing the pedel.

 

[rdezs]

Yes ... And that's where verifying the part number to the application comes in. Very important to compare the part you're taking off with the replacement in all dimensions as well.

Definitely does sound like a master cylinder issue.

 

[cofferson]

from what I read he's replaced the rubber lines and ran abs bleed with a scanner.


being a hydro boost, it seems we just had someone here post about replacing a master and the push rod length being to long. double check that. if your rod between the 2 is slightly to long and it doesn't let the piston in he master completely release. it will cause all kinds of issues.

double check that. needs a bit of a air gap when fully releasing the pedel.

Haha I was the one with the gap. Unfortunately it is the right parts so idk why they didn't fit together neatly. It was ACDelco 18M2564 for the master cylinder and the booster is Bosch 0204777579. I ended up pulling out the rod on the booster and removing material until it was just .030 gapped. Kept it a nice polished rounded tip and it worked nice. A little update though, even though it's ACDelco and it would be strange if it's a bad new master, I returned it for a replacement because no matter what I do with even a pressure bleeder, air makes its way back in the master. Whether it's circling back from the ABS modulator or leaking in through seals I just don't know. Now if I bench bleed this master cylinder and STILL get air back in it once on the vehicle, I also ordered a new abs valve block to switch out if the new master doesn't work or I'll return if it does.. I isolated all four corners yesterday in 2 separate ways and never got a good pedal. First I used a scan tool to shut all of the inlet valves on the abs block one by one which would basically lock the line to that wheel at the ABS. Then I used a line clamp at all four corners. So everything post ABS was ruled out. Then I pulled the master cylinder and it was full of air again.

 

[j91z28d1]

huh.

yeah I got nothing man. not even a good guess.

I hope you figure it out.

 

[j91z28d1]

if you have a helper can you setup the bleeder tubes while the master is mounted on the truck. so you can bleed it with the foot pedal. might help?

I'm just grasping at straws at this point.

 

[rdezs]

I do have one guess related to the ABS system. Actually two guesses.

1. An internal leak in the ABS block.
2. And this one depends on if your model year has TCS, the traction control system. Your original post referenced dragging brakes. The TCS is a rather complex system, utilizing various sensors including a yaw sensor, each wheel speed sensor, and so on. If it detects one wheel losing traction and spinning, it will lightly apply the brake to that wheel, sending power to the wheel with traction. It also does this to keep the vehicle straight, such as black ice. It can make these minor break adjustments multiple times a second, and when working it works fantastic. It's actually utilized on all class 8 heavy duty trucks these days. This goes back to your first post about dragging brakes. If the system were to malfunction it could inadvertently create that symptom. My first inkling would be the ABS block with all the valves..... As in guess number one above... But if your model year has the TCS, it gets more convoluted as you have to consider a bad sensor, or the ECM itself. I would assume somewhere in a Tech 2 it could diagnose it.

 

[rdezs]

..... You might see a referred to as stabiltrack I believe?