Has anybody changed their air dryer filter? Does it really need to be changed? How tough is it to change. If I'm looking the correct piece of equipment under there it seems that there is a plug missing on the underside . I say this because it looks really clean inside this little hole about an inch in diameter on the underside of what I think houses the air dryer filter. Can you tell I'm pretty new at this. I'm trying to change everything that needs it so I cvan start out with everything serviced.