Knot tying gurus and cordage demi Gods I'm in need of tutelage.

I'm looking for a loop to be tied in some Amsteel that when weighted can then be untied easily. Traditionally I'd just use a figure eight knot like when tying into my climbing harness. However when I tied it in the Amsteel, put my body weight onto the knot, the cord was then very difficult to untie. So what knot out there will accomplish what I need?