Portable air conditioner self-evap problem

First I want to say sorry if this is the wrong usenet group but here's my problem: Air conditioner is a 7 year old Sunpentown WA-1210E. The tank fills (and turns itself off as it should) in only 4 hours. I can only drain out half of its 3 pint capacity. It cools fine however. It's never required draining before, except as reccomended at the end of summer. I've googled for tips and have tried adding a small amount of bleach to through the drain plug hole after draining but problem repeats. Thanks in advance for a solution.