03 Trailblazer Major Brake Overhaul - Brake Bleed Problems

I am helping a neighbor out in his attempt to bleed his brakes after changing all rotors, front calipers, pads, etc. This started from him repairing the all-wheel-drive and also trying to fix a spongy, to-the-floor brake pedal problem.

He could not get rid of the spongy-to-the-floor problem so he also ended up getting a new aftermarket master cylinder as well. Even in doing so he still has not corrected the core spongy to the floor problem.

He now is also experiencing what appears to be a pressurized break system keeping at least the front discs still staying locked without any pedal pressure. I have a suspicion that this may be due to still having air in the master cylinder tho' he did do a bench bleed on it. What makes me think this is that it took us over an hour to get the brake lines re-attached to the master cylinder thus allowing the reservoir to drain out. Since attaching the lines we've manually bled and re-bled the system at least a dozen times but have yet to correct this or our other issue of spongy-to-the-floor brakes.

We do have a high end scan tool that can do a ABS Brake bleed but really have no idea what the proper and actual GM brake procedure is. We have been trying to activate the ABS Brake bleed sequence for the ABS module and have only had very intermittent success. Sometimes the pedal goes thru a vibration cycle but a lot of times it doesn't.

I'm thinking that for actually doing the ABS brake procedure right you need to be doing a full pressure bleed with one of those brake fluid pressure reservoirs attached to the master cylinder reservoirs and all 4 bleeder screws open, and while this is being done activate the scan tool to do a ABS Brake bleed.

Any thoughts?

Also, I am in disagreement with my neighbor where I believe that if there is air anywhere in the brake system you will not get a fault code, he believes if there is air in the brake system we should be seeing a fault code. who is right?


Silver Supporter
I agree. Re- bench bleed and connect in a timely manor then bleed calipers like crazy. Abs bleed then bleed calipers again.

Why did he replace calipers in the first place?
I think one caliper did have a seized pin in the front, he opted to change both front calipers.

As far as the master cylinder goes, my neighbor is claiming that after manually bleeding the brakes he's noticing the master cylinder break reservoir pfffts like it's slightly under pressure when he unscrews the cap. Could this also be an indication that there is trapped air in the master cylinder?

As for Fault Codes, even if there are no codes present, the ABS unit still can have air in it, right? (and I assume would be the most probable cause for the brake pedal going all the way to the floor when pressed while the engine is running)

I would like to still have someone that either has the actual GM shop manual procedure for bleeding air out of the ABS system and whether they use exclusively a pressure bleed setup to do so. As well as a GM troubleshooting guide that indicates that a possible cause for the pedal going to the floor is air in the ABS system. I can't convince the guy that it's air in the system that's causing his woes, and he's now just starting to replace parts.

He just put on new rubber brake lines for his front calipers this morning with no effect, and he's been told by another mechanic that looked at the car to warranty the brand new front calipers and replace them. (the mechanic guy brought his own high end scanner and concluded "no codes must equal bad parts".

I can't convince my neighbor that after all the time we've spent trying to bleed the car that we just haven't been doing it right and there is just AIR still in the system.
Ok, still working on this Problem. Spent the last 2 days trying to bleed the system but can NOT got the peddle to keep from going down to the floor. I believe I'm following the bleed procedure correctly and while I haven't seen much, if any air coming out of the front or rear right calipers, the front and rear left calipers consistently have air bubbles (albeit the air bubbles have tapered off considerably).

What hasn't helped is that the trailblazer owner out of frustration keeps changing parts on me, we have bled at least 2 gallons of brake fluid thru the system. He now just had me take off a brand new master cylinder to get a warranty replacement. He also replaced the ABS unit on me with one from a junk yard without proving that the original one was bad. (the original one seems to have paint stick markings on it that makes us think it's a junk yard replacement as well. There are no fault codes, I can get the ABS brake bleed to sometimes work and I reviewed a youtube video where someone did a screwdriver test on a ford truck with an identical ABS unit checking to see if there was movement in the holes under the 2 rubber caps of the ABS unit when pressing the peddle. I confirmed the unit installed now didn't have any movement so I assume it's telling me its good.

I am still not having much luck getting OTC Scan tool to properly cycle the automatic brake bleed sequence. I can get the ABS solenoids to stutter the brake peddle only about a 1/4 of the time. It seems to me I have to have a considerable amount of foot pressure to actuate it properly and most of the time my foot is all the way to the firewall thereby keeping me from applying enough pressure.

So I get to go back and start from square one again tomorrow rebleeding the system without having much confidence I'll be successful. Does anyone have any suggestions or ideas of what I can do to increase my chances?


Well-Known Member
Sadly, some people are just beyond help. Plus, I would certainly question the experience of any mechanic that tells someone that no codes means bad parts. It is certainly possible for a new caliper to be bad, but unless he bought the cheapest Chinese-made part on the market, it is not likely.

When changing the master cylinder, one MUST bench-bleed it before installation. As others have said, it is not hard.

Then bleed the brake lines, starting at the one farthest from the master cylinder and working closer. If using the two person method, make sure the bleeder screw is closed before the pedal comes up. If using a suction or pressure bleeder, the best results are with a pressure bleeder, because a suction bleeder can actually draw in air through the bleeder screw threads.

Once all the air is out (and it may take a while) pump the pedal a few times, start the engine and then pump it more. It should feel soft but should NOT go to the floor. If the brakes are working but still feel a bit soft, skip trying to bleed the ABS unless you have a Tech II tool and just find a gravel road. Do a few stops to engage ABS, and the pedal will firm right up.
Chickehawk, the Master Cylinder WAS bench bled, and now will be bench bled again.

We do have a Tech II type scanner, it's an older OTC Genisys updated to system 4.0 software. Just can't get it to consistently fire the automatic brake bleed sequence. I personally feel it is because the pedal is going to the firewall and I cant press with enough pressure to either initiate the sequence properly or keep it going once it does start.

I do believe I have been consistently able to get more and more air out of it and I may be almost to the point of getting air that is not trapped in the ABS unit out... but we have put A LOT of fluid through to get to this point, at least a couple of gallons worth. But do I have a properly working ABS unit???? Could that be part of my problem? I have no idea. The brake bleeding has taken hours. It doesn't help that the owner is freaking out about all the air that has been in it (almost all of it only on the drivers side - haven't seen any on the passenger side in a while).

The owner has the belief that the air is coming from somewhere, but I don't really think so. The amount of air is consistently smaller and smaller, but the owner wants to shift gears all the time and try something different.
Does it really go to the floor? When I first worked on my TB, I also believed that the brakes were soft. To the point that I brought it to the dealer to have them bleed it after replacing the MC . The brakes on the earlier trucks are softer. It is believed it's because of the aluminum calipers.
Maybe, and this is a big maybe, the rubber lines have gone soft? Maybe at some point the wrong fluid or oil was added to the system? That has been seen here before which requires the replacement of everything except the hard lines.

Some things to do:
- add some Teflon tape to the threads on the bleeders to limit re-entry of air that way
- tap the calipers with a hammer to shake free any bubbles
- push hard on the pedal to forcefully bleed the fluid out
- close the bleeder just before the end of fluid flow at each bleed
- the last time on a bleeder, just do a quick blip instead of a full bleed.

Those are pretty much all of my tips.
Here are related GM-SI printouts.



Silver Supporter
So, uh, the calipers are on the correct sides with the bleeders at the top, right? Check all four. Have seen/heard multiple stories of that issue.


Lifetime VIP Supporter
+2 ...Amen, Brother...

@rmcard ... Do you want an example to show your "friend" that he can understand? Stand right in front of him with a Fresh Bottle of Water... Unscrew the Cap and take a few big swigs... Then put the cap back on tightly and turn the Bottle Upside Down. Then point at the cap and say, "Imagine that this is the Bleeder Valve(s)..." Then point to all of the Air that has collected inside of the very top of the inverted bottle and ask him, " Do You See this? This is what happens when the Bleeder Valve is NOT located at the very TOP of the Brake Caliper... How can you get to all of this Trapped Air to remove it from the Bottle? You Can't!" You’ve got your ‘Left Shoe’ on your Right Foot here... And your ‘Right Shoe’ on your Left Foot here... so to speak!”

If this does not help him to understand what you are up against... Nothing Else Will.

"When the Student is Ready...The Teacher Emerges..." Confucius
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Well-Known Member
Pressure bleed system. if it was drained during rebuilds/replacement the ABS unit will get air in it. By pressure bleeding, all air is removed in whole system

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