I have searched a bunch of threads here and re-bled my brakes for a whole liters worth of ATE SuperBlue (after replacing the existing fluid with an initial liter) trying to firm up my pedal. My mistake, I believe, was taking my front calipers entirely off the car (in order to paint them) and completely draining any brake fluid out of them. I have a Motive power bleeder that works really really well, and is air/fluid tight.

Here is the process I have been through.
1. Removed front calipers, drained them of old fluid.
2. Re-attached calipers
3. Added stainless steel brake brake lines front and back
4. Bled brakes until no air bubbles were coming out from any corner
4. Reconnected everything, noticed really spongy pedal even while ignition was off.
5. Took apart rear calipers and discovered brake pads were not correctly hooked up to the piston. Reattached pad to piston.
6. Much better, but still soft. Rebled brakes and found a few little air bubbles.
7. Brake pedal very very firm, I can stand on it with the ignition off and it does not compress.
8. Turn car on, brake pedal sinks an inch or two and feels squishy.

Like I said, I have rebled this car with a whole liter of ATE SuperBlue. I really don't know if there is any air left in the brake system. It seems strange I get a really firm pedal when the car is off. I now have a few hundred miles on the car and the pedal has improved somewhat. However during testing the past few days the car just won't engage the ABS on the front tires. I can feel the ABS working just a tiny bit. I am really having to push hard on the brake pedal to make this happen, and the pedal itself has traveled quite a bit at this point.

Today in the rain I tried activating the ABS and I barely could do it. I am thinking now the car is rather unsafe to drive, so I need to figure this out.
I think somehow I must have some air trapped in the front calipers from when I drained them completely.

Any thoughts on what to try next? I read somewhere something about "bench bleeding" the calipers if they are drained, I am off to Google Search that term now since I have never needed that before. I also read something about engaging the ABS controller while pressurizing the system. I have spent plenty of time of the years repairing my own brakes and this is the first time in 5 or 6 brake jobs I have been concerned about my brakes afterward.

 

Any chance you got the right and left calipers mixed up? Are the bleed valves on the top or the bottom of the caliper? If thery are on the bottom, I don't belueve you will ever get the air out -- should be on the top.

 

It's unlikely that the air is in the calipers. Try pressure bleeding the brakes again, starting with the right rear caliper, then left rear, then RF, finally LF. This time when the bleeder is open on the first caliper, carefully press the brake pedal to the floor. It should stay there. That will empty any persistent air bubbles in the master. If all of the fluid in your system is new, you don't have to discard it once it has bled out of the calipers. Just collect it in a clean bottle and re-use. Once you are done pressure bleeding, you might want to do a quick brake pedal bleed with the help of an assistant. Just two or three pump-n-empties at each corner. Sometimes pressure bleeders don't totally complete the job.

Also remember that new pads often are spongy for a few hundred miles.

Finally, the reason that your pedal feels good with the motor off is that you brake booster is not working without the motor running.

 

I thought for a minute that might have been it, but the bleed valves are indeed on the top. I did not know there was a left/right so I just ensured the bleed valves were "high" relative to where the brake line came in.

 

Question - if I press the pedal (but not to the floor) and it doesn't stay there what does this mean? I recall pressing the brake pedal a few times during my last bleed with and without the motor running, but still hooked to the pressure bleeder and the result was the same - the pedal returns to standard height.

I will certainly try again. Motor/ignition on or off during the master cylinder purge?

 

Motor off.

Oops, I mistook clutch bleeding. You will have to hold the pedal to the metal with a long prybar, baseball bat, brick or other such certified BMW tool. The brake pedal is sprung, so it will return. Crack the bleeder, slowly and completely depress the brake, and then hold it in place with something.

 

I actually did this once after painting the oem calipers. Very easy to attach backwards. Drove to dealer w no brakes. Yep, they laughed at me. Learned a lesson though.

 

First check the calipers to make sure they are on proper side and there arte no leaks.
If you removed the calipers and let the brake fluid drip until the fluid had drained from the ABS/ASC or DSC unit you may have a problem with the solenoids and may need to bleed the solenoids on the BMW tester. You need to bleed your brakes in the proper order too or you can also have problems.
I have also seen the blue brake fluid make Clutch/slave cylinders and master cylinders leak internally and cause a low and spongy pedal.