As far as I know there shouldn't be a rad cap. The filling is all done via the header tank.
Sounds like it's got either a small pin hole leak or an air lock somewhere.
Get it upto temp and check the rad for any leaks and the hoses for any leaks. These tend to only show when the system is upto pressure.
If no leaks the drain the system and refill it same as if you replace the rad.
My top tip is to refill the system is remove the top hose from the rad and fill the system up through the top hose. Also take the cap of the header tank.
(It's easy to create air locks filling via the header tank)
When you have filled the water upto the top of the top hose, start the car and wait for the thermostat to open (water will disappear from the top hose and should be now be pumped out the top of the rad).
With engine running reconnect the top hose and top the system up via the header tank. Only fill the car with the recommended amount of coolant the header tank doesn't need to be full of water as it's really just an expansion tank and only fills up when the system in upto temp and under pressure.
This should clear the airlock. Bear in mind when the thermostat opens the water will be hot!
Having just mentioned the thermostat this could also be at fault, I had one that wasn't opening fully and was causing overheating issues.
You can tell if the thermostat isn't working as the water will be not pumped around the engine and coming out the outlet at the top of the rad.