I had my radiator flat and could never get the air out. With out a thermostat or some kind of restrictor it will never get the air out! Ive since moved it behind the seats the fill is the highest point, and it works much better. With mine I pull the rear up on the trailer and fill it while buggy is not running(I have drilled out stat) and let it sit. I bought a radiator cap with a big funnel on the top. I fill it and let it sit over night, crank it up let it get hot, and top off. This is a tough problem and will make you consider moving it to the front of the buggy. :****: