Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3931711.

Remixed muzz64's Tire Valve Cap. I removed the MakerBot logo, added a dome top, reduced the size a little from 10.5mmX10.5mmX15mm to 10mmX10mmX14.3mm. The cap can fit a standard o-ring if you're missing it, or you can try printing the one I whipped up.

Printing the Tire Valve Cap

I recommend printing the Tire Valve Cap at .1mm layer height, either 2 shells at 100% infill with or 4 shells which should make it solid. I've had no issues so far with PLA, but you may want to go with a sturdier material like ABS or PETG. I have completed a vacation driving 250+ miles both ways without issue. I was using the actual o-ring from the previous cap that had cracked.

Printing the O-Ring

I have not tested the O-Ring as thoroughly, but if you decide to print it I recommend printing the O-Ring in TPU/TPE at .1mm layer height with 2 shells at 100% infill. It's a little thicker than a normal one which should help with stems being pressed in and releasing air.

When inserting the printed O-Ring into the cap, place it so that the first layer/bottom is facing outward as you place it in the cap. You can then either push it in using a pen or other small tool, or screw the cap on which should force the O-Ring into the cap.