The H31 ATE hydraulic brake booster unit is installed on various models such as the E28, E31 850, E32 735 (early model years) and all E32 750.

The Hydraulic brake booster is located behind the Brake master cylinder on the Fire wall in the engine bay.

What happens is that the front main seal in the unit gets worn out and Power steering fluid runs past it and finds its way through the grub screw on the bottom of the unit, and then it drips on to the chasis rail and then manages to make it way to under the wheel archs and make a big disgusting mess.

What we are doing is fixing this is not too difficult and rewarding.

First you have to depresurize the brake system! pump the brake pedal 20 times with the engine off. and the details here
Note, this is from an E28 site!!
So measure the dimensions before you start, but the principle is the same.
And watch the site:
coming soon.... basically a complete rebuild of the Hydraulic Brake booster