Weird Brake Pedal

The rear brakes are self adjusting, all you need to do is get them close enough. if the adjustment works of course.
In my experience self-adjusters are not worth a ****.
They'll get them close but not where they should be.
I've often had to do or chose to do a manual adjustment on rear brakes and I got a way better pedal after doing so.
 
That's interesting. Always thought you had to activate the ABS pump with a diagnostics tools while bleeding to remove air.

Is that with the engine on or off?
I have heard that you had to activate the ABS when bleeding brakes on some systems but in my experience I have never had to do it and have always got a good pedal after opening the system and bleeding.
Although some systems may require it or some circumstances.
 
At any time during your recent adventures (e.g. when the rear linings delaminated) did the brake pedal ever travel significantly further than in normal use?

The first thing that comes to mind from your description is a master cylinder seal issue. The biggest pointer there is the pedal falling while your foot rests on it. Strong panic braking is also consistent of that because it's a sudden spike in pressure and a slightly damaged seal will still hold. Slowly falling pedal is a peak somewhere - oozing from the piston, leaking from a hose, or leaking past the master cylinder seal. While technically it could be an air or fluid "leak" past whatever seal it is, air isn't going to suddenly compress a little easier while you're holding your foot on the pedal, and a big bubble or air would give you panic stop issues too.

Something I don't feel I see mentioned enough - as cars age the master cylinder bore gets polished in the range where it is most heavily used. This makes it smooth and slightly larger diameter. Past that point the diameter goes back to original and there could be surface imperfections. Grind the seal past that, like during bleeding or some weird brake failure that causes the pedal to fall dramatically, you can now get a little tear in the master cylinder piston seal, for which there is no fix but replacement.

I see lots of bleeding instructions, including BMW's own, which say "pump the pedal to the floor". I specifically place a brick or block of wood under the pedal to keep it in normal operating range on my vehicles.
Absolutely! I have not experienced it personally but then again when I have someone bleeding brakes for me I always tell them don't let that break go all the way to the floor just push it down to about an inch before the floorboard or I will do like you do and put about a 1 inch board or something between the pedal and the floorboard.
What you describe can absolutely take out the master cylinder.
 
Just wanted to update this.

Replaced the master cylinder as it was my gut feeling it had failed or was failing. I was kind of lured into a false sense of positivity for my diagnosis while doing the job. I noticed the new master cylinder was very difficult to actuate, needing quite a lot of force while I bled it. The existing master cylinder was very easy in comparison to actuate.

Installed it and very little difference. Maybe a small difference but honestly, it would have been impossible to say for sure.

So I did some more research and managed to download a copy of Pyclip which is a Renault/Dacia diagnostics tool and re-bled the brakes with the ABS pump running. Had a constant small stream of bubbles out of each corner with a slug of dark black brake fluid.

Brakes fixed!

For all of 2 days, I took the car out last night and really anchored on to see how things were. There was a 'pop' from the rear LH brake drum and now there's a pull to the right under light braking. If you pull on the handbrake while moving the rear LH brake drum is making a grinding sound. So looking forward to finishing work today to investigate!
So what did you find with the rear brakes?
 
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