Ive replaced the radiator, thermostat, radiator cap, water pump and flushed the system but my truck is still over heating but only once i put the cap back on

What is your question?

After replacing above mentioned components it still overheats but only when the radiator cap is on its a 1990 chevy truck c1500

What makes this problem better or worse?

When i put the radiator cap on it starts overheating but its fine if its off

How long has your 1999 Chevrolet Silverado 1500 had this problem?

A few weeks