The message
from "The3rd Earl Of Derby" contains these words:

This is provided the water can drain out and dry out?
Option 2 : tie a plastic ball with some string push the ball down the
conduit as far as possible then make sure the string cannot get obstructed
and start hosing water into the conduit,preferably with a good water
pressure or pressure washer.

Flood it and send a goldfish down.

--
Skipweasel
Pay no attention to that man behind the curtain.