[elemetal]

The expansion tank should be open to the atmosphere then connected to the overflow nipple at the neck/cap. When the pressure in the rad gets too high (water is too hot) it presses out past the resticted spring/seal at the cap into the overflow nipple. This goes out the tube and into the catch tank; here's what has to happen to make it work. The tank input/output needs to be at the tank bottom and the line from the tank to the cap needs to be tight and full of water. I typically use a turkey baster from Safeway or something with Tygon tube down around the bottom of the rad and up to the cap. Ziptie the baster to the rad louvers and cap it with a platsic motor oil cap (keeps dust out but lets it breathe). When the expanding liquid goes out it purges all the air from the rad and the overfill first; then when it cools the water contracts and sucks any needed water back past the cap into the rad. Works great and is super cheap.

Got this technique from a Supermoto site and used it on my SuMo and dirtbike with zero problems. It can get trashed and replaced easily and keeps the coolant level at a maximum. Might not need it with a Gasgas but it's way easier than a fan....