Didn't really have an old tank laying around to use.
Not sure why it would make a differance though. The threads really don't
seal it, it's the contact with the rubber gasket in the cap. The cap is not
distorted by clamping, (verses screwing), it to the pvc fitting. The nipple
is hosed into a bucket of water duplicating the expansion tank. Probably
the only differance I can see is you're pressuriseing air through the cap
verses coolant?

Date:    Sun, 8 Apr 2012 16:53:27 -0400
From:    Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: Coolant cap tester

For testing the cap really needs to be threaded-installed like it is
actually used. The valve set is a separate part that needs to be sealed top
and bottom, without distorting the cap.
That is why I suggested using a coolant tank.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Roger Barkley
Sent: Sunday, April 08, 2012 2:26 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Coolant cap tester

Heres a picture of the gizmo I made to test the pressure cap.
https://picasaweb.google.com/101787477383733954114/Vanagon  Cheap Harbor
Freight adjuster/gauge assy coupled to a pvc fitting. (had to turn down the
pvc to fit inside the cap). Clamped the gizmo to the cap with some vice
grips and connected to air compressor, let air in via the adjuster untill it
bled off. Easier to tell when it bleeds off if you connect a hose to the cap
and stick it in some water, watch for bubbles. As I mentioned earlier,
caused me NOT  to use a new cap I had just bought because it didn't open
untill 40#'s

------------------------------