It sounds like you may have an air pocket. Correct me if I'm wrong but when you replace the radiator you are supposed to fill with coolant and run it with the cap off until the engine heats up enough and the electric fan turns on (do not drive just let it sit and run) This allows it to circulate coolant and it will remove the air pockets. Also, caps are pretty cheap and I don't think it would hurt to get a new cap. Even if it doesn't leak the cap still plays a big role- it pressurizes the system and therefore changes the boiling point of the antifreeze/water. So, get a new cap - when the engine is cool remove the cap (not when hot) and make sure there is enough fluid in the radiator - run without cap on till fan comes on - put cap on and fill overflow tank. Should take care of the problem.

 

had the same problem man. its the head gasket. mechanic actually bought a new radiator for me and actually found the leak after finding that my old radiator was working fine. The more you lag, you'll start overheating eventually, more oil, more gas..more fluid

 

Check all the small hoses and stuff. I had this problem and it was a small small pinhole in the hose and it only shot out when I stepped on the car

 

Well, I checked the coolant level this morning, seems fine. I agree with you that there is some air in the system, causing the loss of coolant. The new radiator seems larger than OEM one.
My fan is not on actually and I do not know what's wrong, but the engine never overheated always below the half way. But I do want to find what's wrong with my fan.

 

ok, bleed the cooling system. do this by removing the radiator cap when the engine is cold, starting the car and letting it run until the cooling fan turns on twice. make sure your temperature selector is at the hottest setting. you can speed this up by reving the car up to 1000-1500 rmps once or twice while you wait for to come on but do not hold it for very long at those speeds. once the fans turn on, you know the thermostat is open and allowing the coolant to move through the system. the coolant level in the radiator should be to the bottom of the neck. if it is, fill the overflow tank and watch the level.

if the fans do not come on, you need to check the electrical. when you turn a/c on, both fans should come on when the car is at operating temp. this will let you know that the fan relays and fans are in working condition. then you should test the fan switch(turns on the fans when the coolant reaches a certain temp) for proper operation. the relay is located in your under hood fuse box and the fan switch is located on the thermostat housing. if you have a digital multimeter and a service manual you can test these using the flow charts in it. let us know how it goes.