I have an older horn, and it seems to be leaking out of one of the valves probably the third valve.
The horn has a good seal. I can seal the bell against my leather coach, hold the spit valves tight, and not blow through it. Too my surpise it even forced air back at me when I stopped. I tried it with all the valves down too.
Any ideas, I think it happens when I finger the valves.