Instead of using an air cylinder, could you just put a block of steel with an adjustable bolt above the drawbar? Set your system up so that the zero on the z axis is offset. Now, when you move the z axis all the way to the top, the bolt will press on the top of the drawbar, compressing the spring and releasing the tool. You would have to give up a small amount of travel in the z direction, but you would get a simple way of releasing a tool.
You might want to have an extra limit switch that stops the motor rotation. It would be bad news if a gcode bug could cause the tool to release.
Ken
Kenneth Lerman
55 Main Street
Newtown, CT 06470