I recently changed my car radiator due to obvious leakages but noticed the radiator reservoir is usually filled up with coolant by the next day with a noticeable reduction in the radiator coolant level. My mech advised I change the radiator cap which I did but the problem still persists although I experience some difficulty when trying to use the new cap. Kindly help in solving this problem good people.

Second phase of a dead head gasket, I'll advice start saving money for repairs or engine replacement

Then I should change the head gasket. Thanks boss.

And the confusing part is that I didn't have this issue till after changing the radiator.