You can log when the connection is acquired and released, that will give you good picture of when these events are occurring. You can use AS7 console application to watch how many connections are in use at any given time and ability to flush the pool on demand.
Run your testcase and watch these and you should be able to figure it out. If needed use profiler and see who is holding on the zombie connections. As per I know there is no such thing as tools for finding leaks, you need to use good old debugging techniques.