Polyethylene airline we sell here has been selected to work well with all of our pneumatic fittings that accept 1/4”, 3/8”, and 1/2” airline. It is black, semi-rigid, and very tough. Very inexpensive but will crush if you bend it in half or step on it. Use the 3/8” or 1/2” inch size as main line from the compressor out to props and cylinders. Then branch off using the 1/4” airline. Or use 3/8” directly to cylinders that have 3/8” ports and 1/4” airline to cylinders that have 1/4” ports.