Or do what I did...I bought a tool for under $20 off the Mac Tools guy that you connect/screw onto a garden hose, then insert other end into a heater hose and clamp it on. Disconnect the other heater hose line on the core and turn on water. It has a valve stem built into the tool. Put your air chuck on it like you're filling a tire and watch the water/rust/gunk blow right out the other core line. The air agitates the water and forces it out. Works great and was cheap...one of the best, reasonably priced tools I ever bought!