Don asked what Bentley said about testing the expansion tank cap.  I don't 
have the Bentley here, but I just went through the process, so I can 
paraphrase what Bentley says:

There are three functions:

1) Seal the expansion tank.

2) Allow pressure to escape out the hose at about 13 PSI.

3) Allow pressure to enter at about -0.3 PSI.

(I don't remember the PSI's exactly, but my numbers are in the range.  My 
item numbers may also be in a different order.)

Bentley says only #2 can be checked, and this with a radiator cap tester. 
 Of course with the VW threaded design, no standard old US tester will 
work.

I say #1 and #3 are the easy ones to test!

1) Observation.  If this is bad, you will see it leaking.

2) As someone else said.  Take it to the dealer and be polite.  The
   dealer can test this.

3) Also mentioned before:

   a) Suck on the hose connector.  It should act as if it is sealed.
      This is also a partial test of #2.

   b) Blow on the hose connector.  It should leak.  The old type does so
      gracefully, the newer type does sound like a duck!

My recent failure symptom was transfer of coolant from the expansion tank 
into the coolant overflow tank when I shut the vanagon down when hot. 
 Worst case, it occasionally overflowed the coolant overflow tank.  I 
concluded that it was the cap's failure to hold pressure (#2 above).  I 
checked the cap as above and it failed #3A.  No need to have the dealer 
check for #2.

Mike

Michael A. Radtke  -  Z74               Voice: 602-862-4897
Bull                                      FAX:         4853
13430 N. Black Canyon Hwy.
Phoenix, AZ  85029                      Email: m.radtke@bull.com