Not a pool person, but sounds like a catastrophic Failure of a PVC pipe.. as in totally severed../ cracked completely. otherwise it would not drain out that fast..
.. no way to fix something like that except replacing it, and whatever cement work that entails.. but hey maybe there is.
Have you tried pressurizing the Line ? .. (which will be impossible obviously if its completely cracked)..
Just simply use a shop vac exhaust port , like when they blow out the lines ( I know you dont do that in Warm Climates like we do in the North here for winter) with your hand cupping the pipe fast , you should give you some back pressure, or not at all , and then you know at least that the pipe is completely severed..
If it were a small crack and you can get to it, sure, you could Epoxy it.