Happened frequently to me before I realized that the cap wasn't on tight. I guess a weak cap could do the same. One symptom is...are the hoses collapsed after the system has cooled down? If so, the cap needs to be replaced.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car