Kris, the brake booster unit of the 750 is hydraulic and runs with Pentosin, same like the very old E32 730 and 735. Later they used vacuum. Hydraulic brake booster units are on all E32 750 and E31 850.
A guy next door from you, Max Fretter had modified his old 730 from hydraulic H31 to vacuum,
you can see that on my website far down under Max's website, maybe that helps.
But actually you were talking about the master cylinder, of course these things work together, so Max's info might help.