I have a friend who's Ziroli Panther was reaching pump limit, and the plane was flaming out. The UAT was a plastic one, maybe polyethylene, looked like a little water bottle. I don't know the brand. Getting a Cat UAT fixed the problem. Also, on a different plane, I had a tank fill issue (was taking too long to fuel) and in the process of fixing that I noticed some gum and slime in the tank, and around the clunk. The cause of that was fuel reacting with brass (perhaps some zinc and/or lead alloyed into the brass). There is little choice in this, except don't use a bare brass clunk. Get a chrome plated one. The tubing is something we're stuck with because chrome and nickel plating would just crack when you bend the fuel line to shape. Bottom line is to maintain the tanks. Here's a pic of the gum. Kind of reminds me of what was found in that screen earlier in this thread.