I assume that you have verified that the master cylinder reservoir is topped up and that you are using a manual bleeding method. My guess is that the brake fluid level in the master cylinder reservoir got down to the point that air got introduced into the system as you were bleeding. You have to verify that the master cylinder reservoir does not become less than half full or air may be introduced into the system which usually will result in the bakes not bleeding properly. Also make sure you replace the reservoir cap each time you check the fluid level or add brake fluid. As you begin to bleed the brakes fluid will flow into the brake lines and as a result the reservoir level will being to drop. Here is a link to the manual bleeding procedure if you do not already have a copy - make sure you follow it step by step. Hydraulic Brake System Bleeding (Manual) — 2012 Chevrolet Impala V6-3.6L Service Manual | Operation CHARM
Hope this helps you.

 

I assume that you have verified that the master cylinder reservoir is topped up and that you are using a manual bleeding method. My guess is that the brake fluid level in the master cylinder reservoir got down to the point that air got introduced into the system as you were bleeding. You have to verify that the master cylinder reservoir does not become less than half full or air may be introduced into the system which usually will result in the bakes not bleeding properly. Also make sure you replace the reservoir cap each time you check the fluid level or add brake fluid. As you begin to bleed the brakes fluid will flow into the brake lines and as a result the reservoir level will being to drop. Here is a link to the manual bleeding procedure if you do not already have a copy - make sure you follow it step by step. Hydraulic Brake System Bleeding (Manual) — 2012 Chevrolet Impala V6-3.6L Service Manual | Operation CHARM
Hope this helps you.

Correct, I have been making sure the master cylinder reservoir is topped off, but it is entirely possible that I made a mistake down the line and let it drop a bit too far. I was panicking that we blew a seal in the MC or something, but I don't think that's the case. Like I said, I have some familiarity with brakes, but I'm trying to KIS with the more rational solutions first.

We'll give it another go once we both have time away from work. I appreciate you sending the manual! There are a few small details that I've missed out on, so we'll follow this and see if we can resolve the issue.

 

Not hard to miss a step or just plain miss something when dealing with newer vehicles. I can remember when I was trying to bleed the brakes many years ago on a 1986 Parisienne - first experience with a master cylinder that had a plastic fluid reservoir rather than the cast iron style with a clip on metal cap. It didn't help that I was tired and the lighting was poor but the result was when I unclipped and removed the plastic top of the reservoir I missed the fact that the rubber gasket was still sitting down inside the reservoir on top of the brake fluid area. Topped it up a couple of times and could not understand why the brakes would not bleed as I kept putting the top back on. Finally clued in and after removing the rubber gasket so the brake fluid got to the master cylinder things went much easier - everyday is a learning experience.

 

True that, I learn something new every time I do some work, even if it's a routine thing I've done before!

We finally got around to re-bleeding following the procedure you sent and.. no dice. Went twice over everything and ensured there was zero air coming out of the lines and the pedal was still spongy. I had some brake power, but it was still all the way to the floor. After multiple bleeding attempts and extra hours of work, I think something else is amiss.
Ended up bringing it in because I don't really have much time to do further troubleshooting at the moment. The shop attempted to bleed the lines, still with no air coming out, and came to the same conclusion. Its tough to say what may be wrong, but I suspect it has something to do with the MC or booster.

This is making my head spin, so hopefully they have an answer soon.

 

Sorry to hear it would not bleed properly for you. Look forward to hearing what the problem was when you get it fixed.

 

Appreciate it! Got word from the shop: they think the reservoir dropped low enough during my bleeding (it did, initially) that the MC ran dry and I messed up a seal during my first pass at it. Tough lesson to learn, but now I know.

They were trying to charge me 1k for a replacement, so I just ordered a new MC and am gonna go from there.

 

Perhaps if you can get your hands on a scanner with auto bleed, I replaced my rear Calipers, when tried manual bleeding fronts were strong rears were just a dribble. Read about scanner, had a friend who had one, tried it fixed problem. When using ya have to wheels in order..LF-RF-RR-LR, had to do it about 4 times, finally rears flowed strong. Good luck

 

You indicated the spongy pedal going to the floor with the engine on....what happens when the engine is off? Is the pedal firm w/engine off? If so, that suggests the brake booster might be the culprit....
Assuming you and or the shop have bled the brakes properly, then either the MC or the booster are your targets.....

 

In my 25 yrs experience a bad booster normally makes the pedal high and hard but no stopping power,but there can always be exceptions.and i have had vehicles that had been leaking for a bit and i had to go to fronts for a couple bleeds then to rears over and over to eventually get pressure to the rears.usually because of proportion valve..best of luck man

 

Perhaps if you can get your hands on a scanner with auto bleed, I replaced my rear Calipers, when tried manual bleeding fronts were strong rears were just a dribble. Read about scanner, had a friend who had one, tried it fixed problem. When using ya have to wheels in order..LF-RF-RR-LR, had to do it about 4 times, finally rears flowed strong. Good luck

I do actually have a scanner with auto bleed! We did in fact use the ABS bleed feature in the order you listed, followed by an additional manual bleed, but no dice. Appreciate the tip though!

You indicated the spongy pedal going to the floor with the engine on....what happens when the engine is off? Is the pedal firm w/engine off? If so, that suggests the brake booster might be the culprit....
Assuming you and or the shop have bled the brakes properly, then either the MC or the booster are your targets.....

The pedal has very slight resistance when the engine is off, but still goes all the way to the floor.
At this point I'm absolutely certain the brakes have been bled as they should, so I'll start with the MC and see if the issue can be resolved. If not, it's booster city.

 

Copy. MC is your prime suspect at this point.....

 

With these cars air does get trapped between MC and where the lines attach at the master... When bleeding the system alone esp gravity bleeding is the best with the car NOT running... If you master ran dry it wasn't a problem before ..so fill it and bleed it ..bench bleeding is best
When any rear caliper work is done you must manually adjust the internal parking brake on the car before depressing the brake pedal .. Bottom line l would take the master off bench bleed it and gravity bleed all the lines starting at RR ..LR..FR..FL... If you have a soft pedal after have a helper pump pedal 3 times and crack each line at master open separately with helper holding down pedal after pumping 3 times each line.

 

I had a similar issue when I had all 4 rubber brake lines replaced on my 09 LT.
As well as all 4 calipers, struts, rotors, brake pads and both CV axles.
Could not get a decent pedal feel doing the manual bleeding.
I took it to get an alignment and asked the guy to do a power/pressure brake bleed.
Everything was great after he finished.
He told me that air gets trapped in the two metal lines right near the master cylinder and sometimes you just cannot bleed them manually.

 

Well, I'm at a loss. Put in a new MC after bench bleeding it, repeated the hydraulic bleeding process, ABS bleeding, and hydraulic again. No air in the lines, new MC, still absolutely zero brakes and no leaks. Really don't know what to do from here, kinda want to just take a crow bar to it with how frustrating this is after hours and hours of time wasted.

 

Sorry to read this. If you want you could try loosening the lines at the master cylinder one at a time just a smidge (finger tight) and then see if you get brake fluid dripping at the loose line line. If you do then tighten the line up and move on to the next one. This should insure you have no air trapped there and then perhaps open the bleed screw at a caliper one at a time and see if you can get brake fluid dripping there and if you do then move on to the next one. Make sure brake fluid in reservoir does not get low while doing this. Just a suggestion to try.

 

Not a bad idea. We have cracked the lines and saw quite the drip, but it never hurts to try again. My last two thoughts are pulling the MC off and seeing if the piston is stuck and maybe just letting the whole system gravity bleed for a long time tomorrow.

 

I don't believe the booster has been exonerated.....based on the symptoms reported here, that would be the next suspect I would closely examine.....

 

You replaced two parts caliper (rear) did you adjust the internal parking brake before pushing on the brake pedal ?? ...As explained in POST 12.
Check your other rear caliper for a leak by parking brake lever ...there is a seal behind it which is hard to check but doable.
READ POST 12 AGAIN

 

Watch this video ....it will help you

The difference in your system is there is no check valve ..instead there is a brake booster pressure sensor which usually lights the check engine light and sends a code when problems appear with it.
You might want to check pending codes.

 

Problem resolved. This one is a bit of a tough pill to swallow, but the root cause of the problem was me, but there's no real shock there. The calipers, pads, and MC I put in were good, and I bench bled the MC properly... HOWEVER, I swapped the lines going from the MC to the ABS unit and ran the flex hose incorrectly above the MC. In its orientation, the air could not fully be bled out, and the first shop I brought it to didn't catch that. After a tow to another shop and some actual diagnostic work, they were able to figure it out and scold me for my stupidity.

Lesson learned: Take pictures before removing and replacing parts and, when in doubt, just stop and ask for help. I've learned a lot working on this car over the years, but there is always room for error and something to learn.

At any rate, thank you all for your previous help, one way or another! This sloppy job was a bit disheartening, but I won't let it dissuade me from doing my own work in the future... although I probably won't touch my brakes for quite some time.

 

@Whythehellnot

I've quietly followed this, partly because your posts were all well thought out, yet the trouble seemed impossible. And partly because I wasn't able to come up with any suggestions beyond what others including you had already posted.

Your post telling us about the solution is very cool to see. We've all been there. And I think you're a stand-up person. I bet @29 Chev agrees (I loved Post #4).

I also think you learned a lot about this car and if you see others having troubles I think you could jump-in with others around here who try to help solve from afar. After this difficult situation your brain is already tweaked in the necessary direction!

Congratulations, you stuck with it and made the right moves. Good job!

 

Thanks for sharing the solution...glad it's fixed.

Ya, I have plenty of stories of bogus repairs through the years. They are definitely the "tuition" that we all pay if we want to play w/auto repair, maintenance, modification,....

 

Glad to hear you got the brake problem figured out. Sometimes the best lessons are the ones learned the hard way as they tend to stick with us.

 

Use service bulletin PIC5468D from NHTSA. You're welcome