Often times truck horns, like everthing else fail due to non-use. Rust and corosion and build up occur right at the oriface in the base of the horn itself. Their proximity up top and forward contribute to this as well. Even horns that retain the front shields are not exempt from this and if the covers were gone for any period of time or if in humid climate or not used often, any or all of these factors will cause a failure due to corosion and road crap building up and plugging the oriface in the base. I'll bet if you pull them you'll find you do in fact have air getting to them.
Electrolisis could also be a contributing factor here as well. I'm not sure what the roof is but the Pete is aluminum and even with a good gasket the hardware creates it. I've had horns plugged up before and you can bring them back with some removal and cleaning. If you do be sure to free up the threads at the base of the trumpet so you can play with the pitch when done.
We all know the old attage- check the simplist things first, and, dont forget to exercize the horns, they need it just like everything else! A great job for the kids!
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