Hydraulic Quick Connect Hose Coupling, Plug, ST Series, Brass, Coupling Size 1/8 in, Female, Max. Working Pressure @ 70 F 2,500 psi, Max. Flow Rate 11.36 lpm ST Series hydraulic quick-connect coupling plugs are the male end of coupling sets. Also called nipples, they're inserted into the coupling bodies (female ends) for use in hydraulics and fluid transfer.ST Series hydraulic quick-connect couplings are valve free to provide maximum air flow with minimal pressure loss. The smooth, open bore allows cleaning in applications where the same lines are used for more than one media including high-pressure water, steam washers, and carpet cleaners. ST Series couplings are interchangeable with similar straight-through high-flow couplings. Steel nipples are case hardened for dent resistance.