[Dysan911]

FWIW. I never knew what that lower tank was for and when I removed the coolant reservoir for a header install, I never filled the lower tank up at all. I did notice however that when my engine was warm from driving the coolant in the upper tank was completely full and when it cooled down the level would drop in the upper tank to about the fill line. I assumed this was normal operation until one day on here someone brought up a topic about the lower tank and how the manual states it should be half full. Ever since filling that half full my upper tank is always completely full no matter if the engine is hot, warm or cold overnight.

I suspect it's just an expansion tank and will draw in or push out fluid from the lower as needed to keep the top full 100% of the time.