We only use filters on systems that use a swingarm air tank. Swingarms are steel and will rust, you need to run a filter between the swingarm tank and valve unit. Without this filter, debris can cause any solenoid valve to clog or leak requiring dis-assembly and cleaning. A schrader valve located at the bottom of the filter allows for the occasional draining of moisture. 1/4" NPT, includes fittings for 1/4" o.d. airline to plumb inline. We do not use any filters on our system with stainless steel air tanks.