You might try inflating a plastic bag to a size just larger than the pipes ID. Once inflated tie it off so that the bag is not taut i.e., you want the bag to be a little mushy. Then tie your string to that and try and vac or blow it in. Sometimes this will work when there is a partial obstruction in the pipe. It conforms to the opening at the obstruction unlike a mouse that will just jam. If the pipe is partially full of water it will conform to the "open" section and actually pass over the water laying in the bottom of the pipe. In this case it may move more slowly through the pipe as it will be pushing a wake as it goes.
I used this once in a similar situation although the run wasn't as long as yours. If there's an obstruction and it stops there you may have to play with the string a little or experiment with the inflation level.
I also use this a lot when I'm on a job and the proper size mouse comes up missing. A sandwich bag works great on smaller pipe although you'd probably want to try a small trash bag in this case. Be sure and trim off the excess plastic.
Good luck!
[This message has been edited by Helectric (edited 01-14-2006).]
