MyE28.com

On my 86 td i have this wierd vacuum problem. It seems the brakes just arent getting the right amount of vacuum at all. I am wondering if anyone had the same problem.
Drift Boss

does the pump itself work properly? Feeding just into a vac gauge it should be pulling better than 20 inches of vacuum and it will do that immediately. If the pump is making good vac, check the condition of the vac lines. No idea what the full setup is on the BMW but there should be a manifold right off the vac pump where it splits to various places. Make sure that isn't cracked and go from there.

Additionally, check the condition of the rubber hoses and the plastic check valve in line with the fat line leading to the brake booster. Mine were all toast, including the check valve. Pump worked just fine, but those hoses wouldn't hold vacuum if you paid them too.

One last thing to check: The rubber grommet in the brake booster itself. This thing was so dry and hard on mine that it literally fell out, still attached to the plastic nipple on the vacuum line. BMW does not sell this as a separate part (which you'd think would violate safety regulations), so you have to go 3rd party. I found one at Napa made for a Toyota that fit perfectly. I wish I kept the part number, but hindsight.

Be aware that the brake booster is a single-diaphragm unit and the brakes will always feel kind of stiff, even when working properly. The big difference is that without vacuum, you can't hold the car on a hill and it feels like you can't slow it down. With vacuum, there's no second thought about any of it, they just work. Pedal still feels really firm, but the brakes work fine.