Brass Inline Needle Valve, Female 1/4-18 NPT Ports, Up to 7.00 GPM & 2000.00 PSI

flow adjustment on this one can be made while the system is still pressurized, so you don't have to kill the line every time you want to dial it in. The fine thread gives you a wide range of control, and once you've got the setting where you want it, the locknut holds it there so it doesn't drift on you. For a 1/4-18 NPT pneumatic setup, it's a solid fit.