There is a vac pipe that has a non return valve built into it and the pipe slots into the brake servo, the pipe (on my 8v gti) runs to the inlet manifold. This allows only flow from the servo to the inlet manifold. therefore does not let any flow from the inlet manifold to the servo. If the non return valve is knackered, then your going to get foreign substances flowing to the servo causing the servo to get damp and eventually pop causing your brakes to fail badly.
NOTE - wether this is the case on the golf i'm not 100% sure but on my previous cars they have all had non return valves, but only failed on my first car (Vauxhall Nova) - BUT (on the nova the pipe ran from the servo to the exhaust downpipe - which when the non return valve failed it allowed exhaust fumes into the servo causing it to pop)
Anyone want to correct me wether the golf mk3 works how i stated....
Cheers.