My coolant reservoir likes to be under the max. Filled it up to max and it's under again in a week. Any information?

 

Leaking

 

... or you had air in the cooling system.
Keep topping off to max line every time the level drops (after engine cools).
After about 5 or 6 top-offs, if level keeps falling you have a leak.
If it stops you had air in system.

 

Are you filling it to max while its hot?

More details....better answers. KnowwhatImean?

 

That's a secondary overflow tank and water will evaporate from there time to time...