But this doesn't always work and O-rings can pop out. The Trap-
Seal, a trapezoidal O-ring from Parker Hannifin Tube Fittings
Div. (parker.com/tfd), eliminates this possibility.  The Trap-
Seal creates a larger overlap between the lip and itself for
increased retention. This reduces the possibility of leaks. The
seal is interchangeable with standard O-rings used in half-
dovetail grooves.