[s-cars] Avant Rear tailgate vacuum leak.....

[David Forgie]

Peter et al: Like the sedan's trunk lid, the rear lock on the avant is activated by a vacuum solenoid.  
On this solenoid, there is a rubber boot. With age and use, the rubber boot splits and leaks. At that point,
the central locking vacuum pump under the back seat cannot pull enough vacuum to open (or close) the hatch lock.

Photo 1: http://12v.org/urs/RearHatchVacuumActuator_Annotated.jpg

Photo 2: Actuator out: http://12v.org/urs/RearHatchLockVacuumActivator.jpg

When I first had the problem on my 98 C4 A6 Avant, I thought it was at the connection to the vacuum system (it has
 the later style vacuum connections) but after buying two new connections (2 on a piece of hose) and them not
 fixing the problem, I finally saw the split on the black rubber boot. "Fixed" with bicycle tube patching glue,
 multiple coats. It worked for awhile but I expected it will fail at some point in the near future. 
(I planned for a replacement solenoid).

It took some months to fail again. In the meantime, I acquired a good used sedan trunk lock vacuum actuator off
a 93 UrS4 that somebody was parting (cost me $5).  When the avant lock accuator failed I removed it (again) and
replaced the ripped (again) black boot with the red silicone rubber boot off the sedan actuator. Works just fine.

As someone mentioned, to get to the hatch lock actuator, you need to remove the hatch trim. Here is a DIY to do that:

http://forums.quattroworld.com/s4s6/msgs/93572.phtml

Good luck

Dave F.