I think those caps are sorta wonky.
A known good used one is worth at least as much as a new one.
Pressure test is a very good idea.
Looking at the caps there are a some variations, and ......I'm sure others
know this very well, but I believe blue versus black refers to pressure
it'll hold. 
I keep several caps to play with. 

-----Original Message-----
From: John Runberg [mailto:jrunberg@mac.com] 
Sent: Sunday, October 28, 2007 3:49 PM
To: Scott Daniel - Shazam
Cc: vanagon@GERRY.VANAGON.COM
Subject: RE: My turn for a cooling sys question

Thanks, Scott. 

It's a new cap (new tanks, too) but doesn't seem to be doing the 'suck back'
feature too well. Next step will be to swap it out w/ a known good one and,
if that doesn't work, preassure test the system.

john

>Sometimes the suck back feature of the pressure bottle cap doesn't work
like
>it should, but it should.