Freightliner air horn issues.

First off, if this is on the wrong board, by all means, mods move it where it belongs.

Now to the meat and potatoes. I have a Freightliner FLD with an under the cab airhorn. Inspected and checked. Oldie but functional and makes money (the truck, not the horn.) A problem sprung up recently (today to be precise). Air horn no longer turns off until the air pressure drops to the warning buzzer. Any suggestions? I'm parked for the week, so its a good thing, but I want to fix it myself if I have to. I can't drive to a shop at the moment without the horn driving me nuts. If it turns out to be something I can fix, I find the idea entirely acceptable. I'd prefer not to pay for the tow truck at this point in time.

So, that said, suggestions, advice, anyone?

 

10-4, will do. Thank you gentlemen.

I will post my results once done.

 

Update.

It turns out that the valve was corroded. I have had some experience doing gas fitting and air conditioning so a bit of flexible tubing and some thread sealing work was not exactly unfamiliar to me. Getting a replacement valve was the order when solvent could not break loose the crud buildup enough for a replacement spring to actually operate the valve. After several hours on the internet and telephone, I found a place that had the right valve in stock and the rest is history at this point.

So, problem is now resolved. Thank you all for the help.