Condesation/oil traps (the glass filters)
I use two at the compressor, and at leastlone paper oneo after athat ft e some a paper filter there (for the moisture not fully condensed before it), and a paper filter nearest my brush as possible for anything that condenses in the lines.
(I use a very large compressor and tank mostly, though I have smaller) It's just easier to unroll/reroll 30 ft of hose from the garage than carry a compressor downstairs/upstairs and put it away.
Sorry for any gibberish... spellwreck insists on it.