E32s have a hydraulic assist on the brakes, run off the power steering pump - not engine vacuum like most other cars. The system keeps a pressure "reserve" in a nitrogen-gas-pressurised steel pressure vessel - a steel "ball" with a rubber diagphram separating the oil from the gas. They age, and then the system does not respond quickly.
Refitting is usually a spin off, spin on, check seals & top-up situation. They're not too expensive.