Smacking / hissing noise when braking

Guys,

it´s an old issue, but anyway I want to care about it. On my 94 BRM there´s a hissing noise when appling the brake pedal, sort of smacking. I guess it´s something with the brake booster, not only the noise (what unusual on german cars), but rather the brake effect comes a bit hesitantly. The first small amout of pedal travel is easy force with this noise, then - once pressure increases - the noise disappeares and the brake effect comes very progressively. Sometimes it´s difficult to meter the brake effort, particulary if you brake very quick, it´s more an unwanted emergency stop. Not really bad, I´ve got use to it, but anyway a bit strange and if I could improve the situation I would do. Any ideas?

No ideas? With Covid no activities on the board?

Try applying the brake pedal while in park to re-create the sound. Once you can re-create it, see if the sound it coming from under the dash or inside the engine compartment.

It sounds like you've a vacuum leak in/around the brake booster.

The brakes are actuated by engine vacuum, so if you're struggling to accurately locate the sound, you can use a stethoscope with an open hose on the end to "focus" your hearing. Another option is a flammable liquid (carb cleaner, brake cleaner, ether, etc) aerosol can with a tube on the nozzle to accurately dispense a bit of fluid. The engine will speed up when you find the leak as it draws in the fluid.

Good luck, let us know what you find!