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.