Kurt,
That faint sound you hear when you release the button is indication that the contact points inside the horn need to be cleaned and adjusted.
When you remove the round cap you'll see something that looks like this....
CLICK Those two nuts adjust slightly up and down and adjust how the horn sounds. You want to adjust them very slowly, 1/16 of a turn makes a big difference. Up or down too much in either direction will result in no sound. There is an electrical contact point that who's gap is set by playing with these adjustments. There must be a slight gap when set right, but not touching.
I find in the two old horns I've rebuilt a light layer of rust built up on these points, and using emery paper I lightly cleaned it up.
Good luck,
Mark
