There a number of different cap designs. Some have a floating center section that does not seal at until it is tightened on the reservoir neck. The only way to test these is to install or use the proper adapter for a test set up. 

Ultimately if you care for your engine at get one of the aluminum tank set ups from Van Caf� or GoWesty. Having a standard cap that can be visually inspected alone is worth the investment. Getting away from sudden tank failure is a bonus. 

Dennis
  

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Bill Marshall
Sent: Monday, April 6, 2020 1:45 PM
To: vanagon@GERRY.VANAGON.COM
Subject: coolant resevoir cap not right

I just got a Febi coolant cap, and something is wrong.  No resistance to blowing in it or suctioning.  Bad cap from the manufacturer?  The inner black portion is also loose, unlike the cap I took off.
Old cap had some slight resistance to suction as it should but held 25lbs or more of pressure, causing leaks in other places.

Bill Marshall
88 Weekender "Clotilde"