Brass Push-In Tube Fitting, 0.38 Tube OD to 1/4-18 Male NPT Swivel, Run Tee

Designed for use with polyurethane, polybutylene, and soft metal tubing, this fitting is suitable for vacuum service and can handle pressure up to the maximum working pressure of the tubing you're using, so it should work well for a small pneumatic system.

The push-in side is for 0.38 inch tube OD, and the male threaded end is 1/4-18 NPT with a swivel.

Polybutylene tubing is one of the listed compatible types, and the swivel male NPT end connects directly to a 1/4-18 male NPT port. The run tee lets you branch off an existing tube run while keeping that threaded connection in the mix. Pre-applied Teflon-based sealant is already on the male threads, so you're not hunting for tape before you wrench it in. The push-in ends handle your tubing without extra adapters involved.