Jay,
I told Steve that he had a problem with his horn pad because his horn worked if he pressed very hard on the horn pad. If your air horn does not work, there are several possible problems, however a horn relay is not one of them since relays are used on electrical horns, not air horns.
The most probable cause is a stuck air control solenoid that is mounted in the LF electrical bay. If you feel the solenoid click when the horn button is pressed, but you get no air to the horns, the valve is stuck or there is an air line off. You can usually un-stick the valve with a good penetrating oil or liquid silicone.
If you use a penetrating oil like WD-40 to un-stick the valve, be sure to blow the valve out because it can harm the valve if left in contract with the seals or wiring, where liquid silicone spray will not be harmful. However the silicone spray is not as good at freeing up a stuck valve.
If you determine that the valve is operating electrically, but you are getting no air and you can not un-stick the valve, replace it. But remember that the most important thing about any repair is to properly diagnose the problem before you replace a part, or you will usually end-up wasting money.
Gerald