In an effort to make my van's coolant system more robust I got one of the GW aluminum expansion tanks about a year ago. Shortly thereafter I replaced most of the hoses and the radiator. My coolant system was anything but robust at this point, I had several leaks. After working on this for many hours I discovered that the pressure cap that came on the new expansion tank was defective, it didn't release pressure as it was supposed to and my system was over-pressurized. It was a frustrating experience, just want to relate this and recommend that pressure caps be tested whenever they are replaced and when there are issues around coolant leaks. Some parts stores like Autozone loan out pressure testers for free that will test the cap as well as pressurize the system to check for leaks. 

-Mark
'87 syncro Westy