If oil was going by a valve that was stuck open then it would just as easily pass through one that is working normally, fixing it would make no difference at all. Fixing the stuck valve is important however because it causes a huge overload on the motor at start up if it is stuck open, BTW how did you determine it was actually stuck?
Running a hose straight off the compressor is just begging for trouble and with no filter/separator I honestly don't see how you ever managed to get any paint to go on without major problems! Washing out the tank is not the solution to your problem, do a search here on air systems and invest in some piping and a good filter/moisture/oil separator. The pipe system and filter/separator are a necessary part of the overall air system and they are NOT optional, simply draining the tank is not going to do much good at all if you don't have a complete system!