Central locking malfunctions when freezing

Hi everyone,

The vacuum actuated door locks of my wife's (yes, I needed to state that) 1998 A Class won't work when temperature is below the freezing point. According to MB's service bulletins this is due to a leaking vacuum component and the fix is to find the leak, fix it, blow each line free of moist and then reassemble it all. That sounds as a LOT of work. However, when the fault occurs, the vacuum pump makes a lot of noise, resembling a small air compressor, and this makes me hesitate to do the prescribed fix - if the vacuum lines are clogged by ice, why would the pump be noisy? Considering its purpose is to make vacuum, a clogged line would only make the vacuum occur quicker, right? My hope is that the problem resides within the pump instead. Has anyone got a clue on what's going on here? I assume the central locking systems among different MB models are similar.

/Jan