I have a 2166 too. They come factory with a very poor cab filter and fresh air inlet system. I removed the side plate on the upper, rear, left side of cab and fitted it with a 2 1_2" dia.X about 2 1_2 steel tubing pipe nipple, connected with flex rubber hose to a nice Donaldson filter mounted on top the cab. Always have good clean air now and never have to clean or replace the dinkey factory filter. If you're still getting dust you must have a leak hole in the cab somewhere or your pressurizer fan isn't working. The pressurizer fan is under the seat and it always runs no matter what you set the output fan speed on the overhead console. Its a good idea to remove the 4 bolts holding seat and tip it forward (detach RH console arm first) and clean AC condensor and check the area for any holes. If your cab isn't being pressurized, you will get dust. Hope this helps you.