Unbleedable Brakes, Coworker's Chevrolet Equinox

Coworker has an Equinox which has had the whole brake system gone through recently- new calipers, soft lines, and master cylinder.  He has run something on the order of GALLONS of brake fluid through it with various combinations of pressure, vacuum, and gravity bleeding.  He has a scan tool that does the stupid GM automated ABS bleed.

Yesterday I tried to give him a hand with it and something is berkeleyed up in there.  Minimal fluid flow, soft pedal, feels almost like the new master cylinder has failed.  Anybody have any cool tricks to try before I tell him to get another new master cylinder?

No tricks. I have had more than one master cylinder bad out of the box. 

Bleed screws on the top of the calipers?

In reply to Streetwiseguy :

Yes, although the rear calipers have a dumb setup where the screw isn't ALL the way at the top so we rotated them up for bleeding yesterday.  No change.

Have you tried bleeding the master cylinder itself?

In reply to Steve_Jones :

He did when installing it, and it doesn't have any of the telltale bubbles in the reservoir when the pedal is pumped.

I'd say bad out of the box master. It's surprisingly common

I had a similar issue with my old Mazda 3 after replacing the rear calipers. Conventional bleeding methods wouldn't work, and I had a friend come over with a scan tool to cycle the ABS pump, but that didn't help either. Ended up getting a pneumatic bleeding kit and that's what did the trick.

Streetwiseguy said:

Bleed screws on the top of the calipers?

Hah! Been there... a caliper rebuild many years ago on a tradesman 100 taught me that lesson.

The issue is that it's an Equinox.

Might need to pressure bleed it.

Similar issues with an S10 (but I didn't have the ABS tool to cycle) and pressure bleeding fixed it.  But, looks like you've already done that.  Could it be the power booster leaking and causing the odd pedal feel?  Not sure how to check it, but might be worth looking in to before replacing the master cylinder.

-Rob

It has been pressure bled.  Lighting it on fire and making an insurance claim has also been suggested.

Bad master, collapsed hose, or improperly bedded pads could all be causing issues.  

Do you get any brake feel when bleeding or is it always stone-dead?

One thing you can try is get some short lines made up and bench bleed the master when its installed in the car, just run some lines back into to reservoir. 

In reply to 93gsxturbo :

Extremely variable- good pedal sometimes, garbage other times.  I believe he also did the on car bench bleed, he has been chasing this for days.

Had a similar problem on my mom's bugeye Impreza a few years ago, turned out to be a failed ABS accumulator.

bad master out of the box.  i see this about 25% in aftermarket parts.

especially the dog-E36 M3 A1 Cardone brand

If the master was a decent brand, Im gonna go against the group and say I think a bad one is unlikely. Now if its a "paint it black and put it back", then maybe.

Assuming you are bleeding it right, I think we can probably rule out air. If you want to try different methods, I prefer two methods. If the system was pretty much left dry during an overhaul, or lost most of its fluid, I life to open a bleeder and put my thumb over it, then have someone else pump the pedal, hard/soft and slow/fast, just whatever just keep pumping it. The idea is fluid and air can push past my finger, but when it stops the pressure on my finger will reseal it and air wont go back in. This is the only thing Ive had work on realy stubborn systems that went dry and I had air way up away from the caliper. The other thing I like is a good old gravity bleed, just crack the calipers and let them sit, keeping the master topped off. Ive never been able to get a better pedal than gravity bleeding. I have access to pretty much all the fancy bleeder tools.

Ive seen many a bad abs accumulator be misdiaged as a master.

For diagnosing, I would clamp off all hoses at the calipers. If the pedal is hard now, you have a caliper/slide issue. If its still soft, you have an upstream issue, probably accumulator or master. If its hard and you need to find the source of the problem, unclamp the lines one at a time, the stiffness of the pedal will drop dramatically when you get to the bad one. Once you find the corner your dealing with, verify the caliper will slide on its pins easily, if it does change the caliper. A frozen slide pin will create a low soft pedal, which seems counterintuitive to some people, but I have seen a few calipers that seem to move easily on the slides, the piston moved freely, and no visible causes, and replacing them fixed a soft pedal. I think it is much harder to get good calipers than masters. I see bad calipers out of the box from even the good brands.

In reply to Opti :

Several gravity bleeds have been attempted with no change.  I made him a loop line to isolate the master so we'll start there and progress through the system the way you've suggested, thanks for the detailed response.

I'll share my idiotic experience with what I determined were unbleedable brakes. I replaced wheel cylinders and drums on my Silverado. Got it back together and bled brakes no air in system. No matter what I did I couldn't get pedal pressure. Did the same. Gallons of fluid through the system.

Real problem? I forgot to adjust the rear drums before I put it all back together. 

GCrites80s said:

The issue is that it's an Equinox.

I've never owned one and wouldn't buy one (they're not my thing) but they make surprisingly good rentals.

If the minimal fluid flow is at all wheels then the master cylinder is doo-doo. Or it's the wrong one.