It comes through the vacuum lines when there is a failure somewhere in the central vac system.

I just replaced the one in the wagon this weekend b/c the oil in there (not much, but enough) mixed with dust and made a paste that bound it up. Of course I had replaced the cylinder first, without replacing the lock housing, so I got to dremel it out when it bound up the second time.

Soooo, replace the housing while you can do it easily, but also make sure your vacuum system is in good working order!

good luck!!!

__________________

Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT