Product review (4 stars): An inexpensive, simple but effective, alternative to a purge pump.
This kit is not specific to evaporative coolers; in fact, the directions are for using it as a waterline for something like a refrigerator.  However, if you're maintaining your own cooler, you'll know what to do (cut the tube between the pump and distributor, insert tee, attach drain tubing).  The included tubing wasn't long enough for my application - I had hoped to use the bleed off water to irrigate my trees.  I already had an overflow/drain tube and an old hose, so now the bleed off tube is threaded a foot or so into a hose connected to the drain plug.  Not the most attractive solution, but it works.