A universal self-locking push-to-connect plumbing insert assembly provides flexibility to an installer to use the assembly in the field with a variety of different plumbing fittings for coupling conduits to such fittings. The assembly comprises first and second interlocking cartridges holding a dual function gripping ring. The gripping ring has inwardly projecting tines for gripping and holding a fluid conduit and outwardly projecting tines for holding the assembly within a plumbing fitting. A sealing ring is captively held by the first cartridge and is positioned to seal the insert assembly to a plumbing fitting when inserted therein and also seal a conduit extended into the assembly. In one embodiment, the assembly further includes a release ring held within the second cartridge for selectively engaging the inwardly projecting tines of the gripping ring to allow a fluid conduit to be removed from the insert assembly.