The new cap should release at about 20 psi and that can pop the cap out of your had. Mercedes engines have been known to hold pressure for days (I have a 190D 2.5 that will pop after sitting aas long as three days - never looses fluid - never any cooling problems, but has occasionally exhibited maintained pressure duruing the 5-6 years I've owned it). Unless you have consistant symptoms of cooling loss or overheating (after the system is completely burped) I wouldn't worry. Occasionally a brand new cap can be defective.

Marshall