I see no reason why it works when attached to the compressor, but won't work when separated..
That makes no sense at all.
I did mine, used a piece of cheap clear hose, attached it to the compressor.
wait a minute,
I had to attach something to my compressor a nipple I think,
then the hose to that,
with the other end going to the trumpet.
Did this dealer put a nipple in the compressor or just shove the hose down in the hole and hope for the best.
I put mine in with JB Weld, then stuck the hose on it, used a clamp to make sure all the air went through the hose,
and I got all the horn anyone needs.