squeaky noise at brake pedal- lubriacte the brake booster?

[werminghausen]

I have this squeaky noise at the break pedal.
First I thought is is coming from the footwell but I am convinced now it is in the internal linkage of the booster.
I know others have the same problem but I never saw someone suggesting a solution.
My question: can I pull the vacuum hose and spray Silicon spray into the booster? Does this hurt the booster?

I am desperate and close to changing the booster if there is no way to deal with the old booster.

[Kalevera]

There's a bellows and a vent on the pedal/cabin side. Probably has dust in it. There are also bushing inserts on the pedal bolts that are made of plastic, a replacement or a good cleaning/greasing can go a long way, but it can be a lot of work. The booster itself is a pressure differential with vacuum and gas filled chambers. Not much in the way of moving parts, more likely that the membrane will break than anything else failing on it.

[werminghausen]

A couple of people have reported the very same problem with the pedal noise especially on German forums, but with no serious cure.
Most say to spray WD 40 on the booster...but this is nonsense because there are no moving parts outside.

The squeaky noise is definitely NOT generated by the pedal inside the cabin. I have tested this for a long time. Pressing down the pedal is moving the piston of the master cylinder crossing the booster. The noise must be generated somewhere in the booster area and transmitted through the fire wall back to the cabin.
The very same noise is audible actually on the engine side of firewall if not louder.
I am ready to take the master cylinder and booster off to solve the problem.
Any suggestions?