You know when you put food in your mouth and it makes your mouth water(saliva) I pretty much think of it the same way. I don't really think it that the bit is working cause a unhealthy horse can have a working bit in their mouth and not have foaming. Its just natural for your mouth to liquidate when somethign it touching your tongue,glands...
There are many types of bitless bridles to consider if the hackamore doesn't work for you. You could always try a sidepull, and indian bosal, a cross-under bitless, and LG Zaum bridle or a LightRider. Every horse has a preference! Best of luck in your bitless adventures!
Guess I am wondering because when I went to test drive her she didn't respond the way I felt she should. She responded to me like a horse would when you were riding them with a halter. Not something that would make me feel safe and in control out on the ranch or on trails.