The cap should relieve pressure through the overflow hose, yes.

I have once had a coolant expansion tank crack...and it wasn't related to excess coolant pressure, as the system operated properly...just a bad tank, I guess.

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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