I am wondering that also, the Haynes book of wisdom states it must be taken to a BMW dealer to get it done. Last year I changed a caliper on my Saab 900, bled the brakes as normal and it has ABS. Since then I have done about 11000 miles with the car and it has never failed to stop properly..............yet!
"The guy who normally services these bikes told me of a tip to bleed ABS brakes, if the ABS part gets air into it. If you have the diagnostic computer or GS911, you can pump the ABS but if you don't, just bleed the system as well as you can then go for a short ride, pumping the brake so the ABS kicks in a few times, then go back and purge the rest of the air out. Worked well for me.
Now, 2 weeks later, the brake still works perfectly. Strong feel, so all the air must have left the system in the second bleed. I was concerned since I had the hose off for a couple of days while I was waiting on the right hose, so lots of fluid had drained, and certainly much air would have got into the ABS part."
I don't know if it works every time, but it worked for him.
He may well be thinking of Grahams Law. Maybe you would like to read up on it. It's certainly a problem in some industries and applications :- permeation of gases through solids.
Yes, i'm aware of it and understand the principle.
However, if you look at all the other variables involved in transferring brake fluid from a container to the master cylinder, bleeding the system and then using it for x no of months, i would say the percentage of gas effusion through the rubber hoses compared to air in the system, air on the surface of the fluid in the m/c and air entrained in the brake fluid then - as far as i am concerned - has no appreciable effect.
let me know if there is any hard proof, approved research that confirms it or industry papers that prove it otherwise and i'll change my mind.