I know this is old... but I was just looking into this since I work at a liquid cooling store. We sell tons of these and many customers have problems... they really do not work well with most tubing, unless you are comfortable wrestling with them... I imagine many acrylic blocks have been cracked due to this.
They simply DO NOT work with primochill tubing. I found that the 3/8 5/8 fittings work well with 3/8 7/16 tubing... they are certainly a pain though, and quite expensive.