Unable to bleed rear brakes, reservoir bulging

Two separate but maybe related issues. 2009 M3 with 130k miles. The prior owner had the car 8 years and I don't see a record for bleeding the brakes, although the fluid looked decent. It was green and fairly translucent. I don't have a moisture gauge so I didn't check that beforehand.

I started bleeding the passenger rear using a Motive power bleeder and the flow was extremely slow, maybe two drips per second. There was very little effect to changing the psi of the bleeder or opening the valve more. (I've done this process 20+ times on another vehicle.) There are a few hits in the search about this.

I let it run a couple hours and got maybe a third of a liter out. Then I switched to the 2-man approach and got decent flow. But after 7 or 8 of those, the pedal stopped moving entirely. I was stumped so I changed to the driver's rear with the pressure bleeder and barely got two drops out of it.

My question: is there a single hard line going to the rear that might have a blockage? Or could it be a bad master cylinder? I'm going to jack it up again to see if the rear brakes are even engaging from the pedal. The pedal feels fine.

Another issue: at 10-15 psi I can see the reservoir bulging out slightly. At 20 psi it's quite noticeable. The plastic seems to be discolored now from the stretch. I assume thousands of people have used a pressure bleeder on this car without an issue. My question: is it likely that the plastic has gotten too old or weakened? I'm wondering if I replace the reservoir whether I'll have the same issue. Or maybe it's something else entirely.

The only other braking oddity is I took it to autocross and briefly locked up a tire at least once under braking. I had DSC disabled and I might've been turning slightly, so maybe that was enough to throw off the ABS. I don't think I've ever heard it engage.

There are two separate lines running to the back from at least the ABS so that each wheel can be pulsed independently. I've also had slow flow using a pressure bleeder on the rears, not nearly as slow as yours and I usually run right about 20psi. I've yet to get a great bleed using it and have resorted to grabbing a second person to get good results. Did you run the ABS sequence while bleeding? How is your pedal pressure, any ABS lights on?

powerbait wrote

There are two separate lines running to the back from at least the ABS so that each wheel can be pulsed independently. I've also had slow flow using a pressure bleeder on the rears, not nearly as slow as yours and I usually run right about 20psi. I've yet to get a great bleed using it and have resorted to grabbing a second person to get good results. Did you run the ABS sequence while bleeding? How is your pedal pressure, any ABS lights on?

Pedal pressure is fine, no lights. Everything is normal, just no flow out of the rear.

I didn't run the ABS sequence. I think the procedure calls for a normal pressure bleed first, which I technically can't complete. Even with a partner I don't get enough flow out of the passenger rear.

With two hard lines it seems unlikely that there's a blockage in both. So there's either something wrong up front or I'm doing this wrong somehow. I plan to investigate more later this week.

I'd try running the ABS pump, maybe there's some gunk clogging it and that'll help dislodge it. I have a cheap foxwell code reader that I use to run the sequence, it is BMW specific. Pretty sure I got it off amazon a decade ago.
Just noticed you're in FL. Where at? I'm near Tampa, if you are close, you're welcome to swing by and throw it on the lift and I'll give you a hand.

All good! I drove it around a week, activated the ABS a few times, and they bled out just fine with the 2-man method. Maybe something was clogged or maybe my wife was confused about the process.

It's still very slow with the power bleeder and I don't want to give it any more pressure. I'll consider replacing the reservoir when I go to replace all the old lines.

I’ve noticed a slower bleed rate on the rear calipers while using the Motul bleeder. I’ve never had the ability to actuate the ABS, nor ever had an ABS fault code either. Recently bought an Autool (spelling?) and it hasn’t ABS function, so I’ll bleed them at some point. I wonder if the slow rate is because a ABS Chanel valve might be closed or something? Not sure how the ABS module works, just a guess.