The Breather Cap from Buyers Products acts as a ventilator to provide a free flow of clean air into your reservoir. It features oil-resistant filter elements that keep out dirt and dust particles as small as 40 microns. Breather cap sizing ranges from 1/8 in. NPT to 1 in. NPT. Provides free flow of clean air into hydraulic fluid, oil, chemical, and coolant tanks or reservoirs.