2002 Ford Ranger 2.3L Overheating

I have an issue with my truck overheating. Basically what happens is the engine gets hot and "boils" all of the water out through the over flow tank. My truck does not have a radiator cap so you can't fill the radiator directly like I have seen in other vehicles. Now, a little history on it. I have recently had the radiator flushed and repaired because the the above problem occured and I didn't realize at the time that there was not fluid at all in my cooling system. Since then I have also had to replace the overflow tank because the liquid has been super heating the plastic in the tank and had caused it to deteriorate. I am still having th problem of the water "boiling" out of the system. I have checked and there are no leaks and I have no water in the oil or leaking out of the tail pipe. What can I do? Any suggestions are welcome.