NS51 series quick connects allow for connection and disconnection while lines are still under pressure (up to 500 psi). The time tested tubular valve and poppets internal design provide positive fluid shut-off (check valves in both the male and female sections) and permit virtually no fluid loss upon disconnection (as well as minimal air inclusion). When the coupling sections are brought together, projections on the ends of each check valve serve to move the other check valve off its seat, allowing oil to flow in either direction. If the coupling is not tightly made, one or the other of these check valves may not be properly lifted off its seat. Under this circumstance, oil would flow in one direction but not in the other when the pressure is relieved on the feed side. Even with no oil leaks, this dangerous buildup of pressure can occur when the fittings are one turn or less from fully installed. This is a hazard in the mining, construction and industrial sectors, and wherever hydraulic equipment is used. If the connection of a quick-connect coupling is not made completely tight, one of the internal check valves within the coupling may trap hydraulic pressure behind the check valve. The absence of any leaking oil around the coupling is not an indication that the coupling has been made completely tight. These are used in similar applications as our 5100 series, this New-Line brand is an alternate brand to our premium couplings. This style couplers are mostly used in tractor trailer hydraulic connections.