Last day I noticed that there was a hissing sound right after I shut down the engine. Opened the bonnet and noticed that it came from the brake booster.

After some tracking, I found out that there was a leak at one of the nuts that holds the main cylinder to the booster. The leak was due to the nut holding one vacuum hose clamp allowing air to pass the threads.

I placed one rubber gasket in between and that made the hiss disappear from the nut. Instead there is a much more subtle hiss that I haven't been able to trace.

The brakes work nicely and it is possible to lock the wheels on asphalt, however if I brake and shut down the engine, I could feel the pedal press against my foot.

Do I have a leaky booster? I have listened below the steering wheel and it is quiet.

TIA