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how I use the pneumatic fittings is
1 push in the ring to the the rest of the fitting(and while holding the ring in,
2 insert the tube into the hole in the middle of the ring, and push the tube as far as possible into the ring and fitting,
3 release the ring and attempt at moderate (definitely NOT full) strength to pull the tube out if the tube does not move it is then correctly set up.
if the tube pulls out then 1 you either used too much strength, or have a faulty fitting.
the black ring is a control lever for the grip mechanism inside the fitting(basically little thin metal teeth that the ring pushed down (away from the tube)))