1812- I would give a look into siphon drains. Had I found this site soon enough, that's the way I'd have set it up. It is healthier for the pond, but I like it because the pipe that goes thru the dam is near the top surface the pond. If you have a leak in the old style pipe systems, then it becomes a real hassle to fix, as it sounds like you found out.
Are you planning on making the concrete one so you can drain the pond all the way down? You still could do that with the siphon system.
