Peter,
Rare for servo's to fail, but having said that I recently had to replace one on a Skoda (VAG group). The symptoms were that the front brakes would stick on after repeated applications of the brake pedal and would only release after leaving it for some time or, like you, letting the pressure out from the hydraulic line.
Has the 411 got two servo's like my 412, or just a single?
One problem which can occur is that the vacuum chamber can fill with water or petrol over time (condensation). You can sometimes correct this by draining the chamber (means taking it off), but if it has got petrol in it this may have rotted the seals.
Repair kits for some servo's are available, see for example
NORTHWEST AUTOMOTIVE HYDRAULIC COMPONENTS
This site has copies of catalogues which you can download.
Good luck
John P