Sorry to interject a Cobalt problem here, but it's a forum that I trust; never registered on the Cobalt forum, anyhow. Yep, it's a brake problem again, but this time it's the one I've been fighting off & on for two years...my wife's 2008 Cobalt. I made mention of it on other posts, lastly here https://www.chevyhhr.net/forums/brak...18/#post928092

I gave up on it in June, as it was getting too hot for me and my helper, but my wife is getting ready to get her dying mother's (82 yo) 2020 or 2021 Chevy Equinox, so she says it's time to get rid of two vehicles, the '08 Cobalt LS coupe and my '09 HHR Panel LS. I don't want to lose the HHR, so I'll suggest the '98 GMC extended cab that just sits (it's her truck, but in perfect condition, with 182k miles), or reluctantly my '04 Chevy 2500HD WT (it's a low-mileage truck with only one problem, a low gear starting to slip...but tow/haul fixes it). My '01 BMW X5 project car is off the discussion list, as it works perfectly. So, to get them saleable, I need to fix the Cobalt's brakes (uninspected and unregistered since 2021), and put new struts on the HHR (at 165k miles, everything is in great shape, otherwise). I still want to keep it.

I figure that the Cobalt LS's brakes are nearly identical to the HHR LS, so someone might know what's wrong. The Cobalt has new front rotors & pads, new master cylinder (I installed it), and the rear brakes (& handbrake) were adjusted to like-new.

• 1) We (my younger, engineer, helper helped me with the under-car work) adjusted & bled the brakes in sequence three times, and couldn't fix the sinking pedal. That was before I installed a new MC. Afterwards,
• 2) we again bled the brakes three more times, before giving up for awhile (pedal didn't sink all the way to the floor this time, but slowly to 1" from it, after stopping). A month later,
• 3) we resumed working on it, this time using my Foxwell NT510 Elite scan tool (with dual BMW and GM programming) to bleed the ABS system. Again, three times. All fluid was completely replaced as we were using a Motive Power unit as well. We were both unhappy with the results, so that was all until today.

This morning, I looked under the Cobalt once again to confirm that the brake lines, cylinders, and MC/booster had no leaks (they never had previously, either), then took it out on my local drive-cycle (about 1.5 miles around the block, on semi-rural roads), doing all types of braking (light, medium, heavy...but no stomping as my wife does), and the Cobalt would stop, but with the pedal sinking, unless I pumped the pedal once or twice, then the sinking would stop,,,for a second. I drove my old '69 C-10 for two years like that (or even under worse conditions as it had drum brakes, no ABS, and it was in heavy commuter traffic in DFW, or while towing my racecar. I made dozens of stops at speeds up to 70mph, and never a problem...other than the sinking pedal.

Conversely, braking while backing (again many times, at many speeds) was rock-steady, with no sign of a sinking pedal. I have no idea whats happening here, so I'll probably have to get a brake specialist to fix it...at $$$ cost. Any ideas?

 

Rear brakes probably need adjusting.

 

X2 ^^^ if you are absolutely certain the are no leaks anywhere in the system, did you replace the hydraulic flex hoses , up front and out back? A new mc can leak back into the brake booster, speaking of which did you check the power brake booster? Any vacuum leaks ?

 

If you installed new rear shoes, maybe they are backwards. On HHR there are 2 different systems used. The star adjusters turn in different directions in the 2 systems.
The reason I suspect the rear brakes is the rear shoe is the adjuster shoe. I you are going back it engages first and moves to force the "click". Moving forward the front shoe engages .

 

The rear brakes were adjusted where I'd want them, and the handbrake is starting to engage at 3 clicks...fully locked at about 6. Should it be a bit looser? How would that affect the superb braking upon reverse?



The hoses & hardware looked fine, felt fine, much better than one would think, but these cars/trucks in my "stable" aren't subjected to much adversity (except for my wife's driving), but I'll feel the flex hoses for softness or bubbles. I made sure there was no leak into the MC before I bought and installed the new one (the old MC had an internal leak).

 

I was posting my first reply while you had made another one. The rear shoes are the originals, which hadn't been adjusted for awhile...lotsa material left. They were never removed, just adjusted by the star wheels.

after my last post, I went outside and felt the front flex hoses (but couldn't reach the back ones). Firm and dry.

 

I don't see where you bench bled the master cylinder?

 

Since I couldn't reach my vise in the garage (it's surrounded by tons of my wife's collectibles and junk ...same thing, in my mind), I mounted the new MC onto the brake booster, and bled it in place, using a brake bench bleeding kit, before I hooked the lines up.

 

Hoses can flex and stretch under pressure, have some step on the brake pedal while you inspect the hoses , the e brake should be fully engaged at 3 clicks.

 

I meant that it is fully engaged at 3, but impossibly tight at 6!