I bought a bag of 50 blue ones from my local VW dealer years ago.
It was sealed and all were brand new.
I use my Snap On coolant system tester to test the caps.
It has different connections that attach to the tank, radiators and caps.
Mine has a regulator and is air powered, but most have a hand pump attached.
I have seen many make one out of a old garden sprayer and bike pressure gauge.
I believe there is a how to on the Samba forums.

dylan

________________________________
From: Luke Bakken <luke.bakken@GMAIL.COM>
To: vanagon@GERRY.VANAGON.COM
Sent: Sat, December 26, 2009 12:12:35 PM
Subject: Re: No coolant traffic

On Sat, Dec 26, 2009 at 8:52 AM, dylan <insyncro@yahoo.com> wrote:
> I have had a 25% failure rate with brand new blue caps over the years.
> I now only use the black ones.
> Don't know the difference, other than they seem to be better quality.

Hi Dylan, where do you get these caps? I may have run into the same
problem with my setup. If the coolant can't flow to the overflow
reservoir tank, what happens with the expanded coolant? Does it just
leak out at the weakest point in the system?

> Pressure test with 15lbs. to sure it is working properly.

I've read in different places how to pressure test the entire system
... but how do you just test the caps?

Thanks!