1990 BMW 750Li Brake booster/Master cylinder

I have a fluid leak located somewhere beneath what I think is the master cylinder/brake booster. I can't see exactly where the fluid is coming from but it 's leaking directly on the frame in the engine's compartment right beneath the master cylinder/ booster assembly which is deteriorating the paint. I've been putting power steering fluid inside this reservoir located on the passenger side underneath the engine's hood directly in front of this brake assembly. I know this fluid is not the correct fluid but it's just temporary. After doing a little research, I found out that this reservoir holds the fluid that supplies not only the brake booster but it supplies the fluid for the power steering as well as the rear suspension. Before I start removing the brake booster, I was wondering what are the common defects that occurs with this type of scenario and can they be remedied besides buying a new brake booster if that's the problem. My brakes haven't started fading yet but my rear suspension tends to sag when the fluid in the reservoir is low. Is this the common thing that happens when the brake booster is going out or could it just be a loose fitting? I recently had some work done which required the engine to be raised out of the engine compartment a little. I don't know if this leak problem just occurred or if it's been existing because I just brought the car and immediately put it in the shop to fit an oil pan leak.