Hissing from under the Brake Booster (1980 528i)

[brotherhay]

So after starting the car (1980 528i), or just after shutting it off, I can hear an audible hiss from within the engine compartment.

Theres no fluids leaking, so its vacuum related. Coincidentally it sounds like the hiss is right below the brake booster. I believe there is a brake pressure regulator under there? I suspect I need to remove the booster to get to it?

Something needs replacing, thats for sure, just checking with the pros to find out if this is is something common, or if there are any tips or tricks to this one.

[PatinaBeforePolish]

The booster holds vacuum and the hissing could be related to a failed diaphragm in the booster. The hose leading to the intake log is connected to the booster on the lower drivers side and could have a leak as well and cause the noise. There is a brake circuit distribution log underneath the booster but nothing vacuum related.