I have found that this happens when I over fill the overflow tank. It should be about one third full to allow redraw of fluid when the car cools. Also check the state of the radiator cap. I would replace it if it looks corroded because if the over flow valve here is no good you would lose coolant back to the overflow tank and quickly run low on coolant, not to mention less cooling at lower pressure.
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I have found that this happens when I over fill the overflow tank. It should be about one third full to allow redraw of fluid when the car cools. Also check the state of the radiator cap. I would replace it if it looks corroded because if the over flow valve here is no good you would lose coolant back to the overflow tank and quickly run low on coolant, not to mention less cooling at lower pressure.