Vacuum pump or servo

Ive got a 2005 tdi 90hp 1U ocy.

I think the vacuum servo is leaking.

When car is running and i press the brake pedal a few times and it starts to get stiff after 3rd or 4th press, or is this normal?

It also gets get a lot of N18, N75 and vacuum valve fault, but cant find any leaks in that part of system.

I know that a leaking servo has happened to some cars before, does anyone know exactly where they leak?

Any advice greatly appreciated.