Air horns are simple. A tank and a compressor, solenoids to let the air flow to the horn and the horn itself. If you are sure of your air supply and the psi, then check the solenoid, typically they are about the size of a 35mm film cannister (if you remember those) and they will have wires to them; they are electric and will be either close to the air source or close to the horn typically. Do they open when you hit the horn button? If so then does the air flow to the horn?, if so then your horn needs to be cleaned or the diaphram replaced or both. Hadley sells kits for this, and I have heard of folks using other thin metal material to make the diaphram. Most of these repairs are simple;if you can reach the component a bad solenoid needs to be replaced, they are about 20 to 40 bucks each. The horn repair kit is about 25 bucks I think.
IF it were me I would do both as long as I was messing around with this.
I do all my own stunts
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Doc(He's a PhD)