I'm not sure I'm diagnosing this correctly and would appreciate some help. Last week the dash warning light came on for the radiator - showing low on coolant. Temp was right where it should be. I checked the expansion tank and found it was about 1" - 1.5" low. And I could see signs of wetness around the tank's outside mid-section parting-line and some wetness under the tank on the inner fender wall. I think I can see a hairline crack but not sure. So I "diagnosed" a small crack in the plastic tank. A few days later the dash warning light came back on - and this time the fluid was barely 1/2" low. That seems like too small a loss to trigger the warning light. So my questions are: Is that enough coolant loss to trigger the dash warning? And what else could be triggering the dash warning light? What else should I be doing to get a complete diagnosis? Does this call for pressure? BTW I'm pretty certain I replaced this tank about 50K ago. Peachparts lists a newer part number, and MB is only new purchase option they list. Thanks.