The tank is an "expansion" tank. It allows the coolant to expand as the engine warms up. The cap is rated at 2 bar (30 psi), too high in my opinion.
If you fill up expansion tank to the top, then the coolant cannot expand against an air pocket. If you were awake in Physics class liquids do not compress, but gasses do. Once air pocket is reduced/eliminated pressure rises very quickly, then weakest part fails.
There is a reference float in the tank that needs to be CAREFULLY used to verify coolant level. Once float is topped out, you can keep filling tank.
I always adjust float level based upon coolant temp, low float cold, nigh float warm. It you set high float level cold, you have less expansion space.
Last edited by jfoj; 12-06-2012 at 01:09 PM.