Sanity check - vacuum advance

[quk@isham-research.demon.co.uk (Phil Payne)]

Bored out of my skull (ur-quattro not expected back until next Friday) I 
started investigating the general malaises of the SO's Passat.
 
Apart from having the wrong plug caps fitted (they only rest on, but don't clip 
to, the plugs) I found the vacuum advance doesn't hold vacuum.
 
There's also a vacuum-operated switch on the firewall.  It looks like one of 
these "change up a gear" light switches - but I've never seen such a light.
 
The hoses are fine - but the diaphragm in the distributor responsible for 
vacuum-operated advance/retard doesn't hold vacuum _at_all_.  Doesn't even seem 
to try.  I know a running engine can suck a whole lot harder than a MityVac, 
but I would expect it to at least _try_ to hold vacuum.  No deal.
 
This is a n/a I5 "JS" engine.  Anyone had this fault?

-- 
 Phil Payne
 UK Audi [ur-]quattro Owners Club