I put the filler in the upper hose. I run the engine up to temp then shut it off, unscrew the little fill cap, squirt coolant until the hose fills then cap it & run the engine some more. 3 to 4 repeats of that & the heater core is full. I wasn't too clear in that last post but eventually I get the air out. Dealers pressurize the water in to get the air out but I'm not paying for that.