the H31 brake booster unit is supported by (m)
the power steering pump. If you turn off the engine, there is no support.
When you turn off the engine, open the Pentosin reservoir, then pump the brake pedal about 20 times till it gets hard, then the Pentosin container will be full, because the brake booster bomp is emptied then.
Get a Bentley Repair Manual for the E32, good investment, nowdays available for abt. 85 $.
See also my website for further tech tips
SHOGUN'S TRICKS AND TIPS FOR THE SEVEN SERIES
Also change every 2 years the brake fluid.Shogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html
Re: the H31 brake booster unit is supported by (m)
This is why I asked for the production date here in the US the very very early 88 735's had the hydraulic boost system then they went to vac.