I don't see how air in the system could cause this.

How much freeplay in the rear pedal travel before you get pressure?

If we remove the ABS from the equation the simplest answer is an internal brake flex line failure causing a "check" valve holding pressure in hard applications. Since you can manually open the bleeder valve and release the caliper something is holding pressure. The second could be the drop tweaked the rear pedal bend and the turnbuckle adjustment to the master cylinder is to tight and/or sticking and harsh applications don't release the MC enough thus holding the brake pressure.

You don't have any type of "power" ABS like the servo ABS systems so I just don't see it as an ABS fault. You dig that deep after the basics I think I would initiate a few ABS rear brake slides in a safe environment and make the shuttle valves in the ABS block move but I really think if a harsh ABS initiation occurred and the system couldn't dump pressure in an ABS event it would pop a code and the light would be on.

I wonder if you could isolate the rear flex line by breaking the banjo bolt loose on the MC or elsewhere on the system upstream of the flex line. If you vent pressure and release the caliper at the bleed screw and you see it happening, vent the pressure upstream and see if it bleeds off the same. If the line is coming apart internally it won't vent pressure on the up side of the failure as quickly, if at all, as it does at the bleeder.

 

Start with brake pedal. Remove, clean, and lube pivot point. Pedal should return to full up position after being fully pressed, with only the spring pressure. Make sure nothing is interfering with spring, or pedal movement. With pedal in full up position, there must be free play between the pedal and master cylinder plunger. If all this checks out and appeared clean and normal, I would remove caliper and separate the halves to inspect the pins. They should be lubed and not worn badly. No rust. 36k mi. is enough miles for dirt and rust in these areas to cause what you are experiencing. Since you can make this happen in a non moving situation, it is not heat causing the lock. I would suspect pedal or MC issue.

 

Thank you both for the thoughtful responses. Some follow-up questions and more information:

As to ABS: I did test that out, and the ABS did kick in while I initiated a slide in gravel. I then tried to lock up the ABS on pavement, which resulted in the brake seizing up. Interestingly, you would think that the ABS would try to fix that by dumping pressure, but not that I can tell. Also, no ABS error light comes on. I would think it would either dump pressure or it would give me an error.

Free play: when not seized, I get a good inch or so of free play - which seems normal to me. When seized, there is zero free play.

The drop - It landed on the left side, so it shouldn't have affected the brake pedal. That said, I'll end up taking it off and making sure nothing is going on there.

How hard is it to pull off/replace the flex line? I haven't worked on ABS systems a lot and, candidly, I don't understand them. Does the line run through the ABS unit somehow, or if not, how does it release the pressure during a slide? I've replaced flex lines on non-ABS systems before, but don't want to screw this up.

What I find baffling about this is that the pressure relieves itself over time - I would think that if it was a stuck pedal issue OR a flex line issue that it would seize and stay seized. It also seems to be either fully seized or fully free with no in-between state. Also odd.

I won't be able to tear into it for a couple weeks as I'm about to go on a trip to Alaska (by boat, not bike), but appreciate your (and anyone else's) thoughts in the meantime. i'll certainly post an update if/when I figure this out.

Thanks!
D

 

 

What you describe is what happens when the master cylinder does not return to the fully open position, or when a bit of debris gets into the tiny orifice inside the master cylinder that allows fluid to flow back into the cylinder's reservoir when the brake is not applied.

In either case, as the caliper heats up, hydraulic pressure in the line rises, further applying the brake, leading to more heat and more pressure until the caliper is fully locked up. That is why the pedal feels rock hard once you are locked up. After sitting, the temperature goes back down, relieving the pressure as the trapped fluid reduces in volume.

The moving cup in the master cylinder relies on a spring to return fully to the "off" position. If there is crap somewhere in this piston, it can keep the spring from returning the cup to the position that allows the orifice to the reservoir to be uncovered, even if there is slack in the pedal.

The orifice in the master is also tiny, like pinhole sized. If a speck of dirt that is just a bit bigger than the hole size is resting inside it, it can act as a one way check valve.

Either of these conditions could have happened when a bit of dirt or shed rubber found it's way into the wrong spot. I'd pull apart the rear master cylinder, thoroughly clean it and check that the piston/cup is fully returning to open and exposing the bleed orifice.

 

Thanks Snake and Math! I’ll give both of those a try. BTW, is there a source for rear brake lines other than BMW? Spiegler has the front lines for 2013-18 but not the rears. I would assume it’s just a certain length with banjo fittings on each end, right? One to the wheel and another to the ABS?

 

That happened to me when I thought I could adjust the brake travel like every bike I've had before. You can't! Mess with travel and the pads will drag on the disk just enough to heat up the system and slowly seize!

 

Worth checking is the fluid level in the reservoir. If it's too high then heating of the brakes will cause the fluid to expand. Not enough free space in the reservoir could cause the pads to be forced on.

 

An update:

I took apart and cleaned up the rear calipers, per the video posted by Math. There was a lot of crud on the pistons, especially on the exposed end. I cleaned those off good, cleaned everything, put it back together, and that seems to have done the trick. Did everything I could try to get them to seize up again, but they kept working as intended. My theory is that when I put in new pads I pushed the pistons back, so that the crud on the ends pushed back past the seals where it could jam things up. I can now lock up the back tire (in Enduro Pro mode) or get the ABS to work (in Road), and it releases just fine.

Thanks to everyone who shared their ideas, I deeply appreciate the help. Since it seems to be working, I did some adventure riding to test things out. Here's the Bike at 4k feet.

Thanks!
D