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Son going out of town so decided I'd check fluids in 2005 Civic EX. Overflow tank was a little over the high mark. I cracked radiator cap and cool coolant rushed into overflow tank. It did not overfill it. I got my pump and pumped coolant out of tank back into radiator. It filled radiator up and coolant was where it was before cracking cap. Is this normal. I did not think cooling system would be pressurized after sitting all night. I just had fo replace my thermostat andbleed systdm a week ago. I guess this could be good because pressure would indicate no blown head gazket or leaking hose. Someone more knowledgeable than me please share.