Originally Posted by tkamiya

No, that's not normal. If your cooling system is developing vacume, it's a pretty sure sign that your pressure cap is not functioning correctly. If you look the underside of the cap closely, there is a black disk at the middle of it. When negative pressure develop in the system, that disk is supposed to pop open and let the pressure equalize.... I'd recommend getting a replacement.

Several unusual places to check on your vehicle....

In the passenger side front fender well, behind the cover, there is a recoverly tank. There is a over-flow tank in there. If that tank overfills, the fluid will come out of a hole that is in the fender, near the tire. I have not confirmed this myself, but once the coolant goes into the overflow tank in the fender well, when the engine cools, the overflow does NOT come back to the tank where the pressure-cap is. So, if your cap is not maintaining pressure, the level will continue to go down....

Are you filling the fluid to where the seam is, on the tank? You are not supposed to fill it like other cars where you fill it to the rim....

If you look at the washer fluid tank, you'll notice, there are two hoses attached to the cap. That hose is part of the cooling system. There is a valve at the other side of the cap. This is made of plastic. Sometimes, this piece breaks and let the coolant go into the washier fluid. Is your washer fluid rising when your car is hot?

In your upper tank (where the cap is), if you look underside of it, there is a connector... and behind it, a level sensor. This sensor can develop a leak. The first symptom is a wet connector. Eventually, the fluid will contaminate the contact and the light will come on dimly. It's pretty easy to check.