Compression fittings are similar to PTC fittings in a way that they both adapt the airline onto an air tank or other component. So how do these work? Compression fittings use a compression nut to secure the air line to the barb-shaped piece in the center of the fitting. For this to seal properly, the line has to be cut flush so that it fully seats over the barb. Below is an example of a good and bad cut.
The bad cut example shows how the line sits when you press it over the fitting. Half of the line at a minimum is not over the barb, and air will leak out of this connection if left this way.
The good cut example shows how the air line fully seals around the barb fitting when pressed onto the fitting. It is imperative that the line is cut flush and straight for the best seal. A poor cut will not only lead to an air leak, but when you honk the horn it will not sound nearly as good.
Bad Cut ❌