I'm not a registered member here, and don't know if this will go through, but I'll give it a whirl.
Have just been listening to an MP3 of the Vernon Dahlhart recording from the Smithsonian collection reissue. Vernon says "lost his airbrakes." He pronounces it "(h)ahrbrecks" but the word is NOT "average" or "leverage."
I don't know the proper word, if there is one, to describe the vocal affect I'm hearing at the beginning of the word, but I'd call it "aspiration." It's akin to his "fahrmin" for "fireman," where he breathes out on the "ahr," sounding it in his throat, rather than the nasal/glottal whine of "fiiiire" we're more accustomed to hearing in clipped standard American.
The "brecks" second syllable has a kind of a swallowing sound to it. I don't know what that's called, maybe a glottal stop? The swallow prevents the "a" from being a standard long vowel, which requires an open throat.
I know there are technical terms to describe the physical movements of throat, tongue, lips, etc. that make all these sounds, and I apologise for my ignorance in not knowing them. I hope you get the drift of my layman's language.
Anyway, unless there is some documentary evidence to the contrary--something written by Dahlhart himself--I say he sang "airbrakes." It's quite possible some Yankee running dogs can't understand his lovely lilting suthren tones, and therefore repeated him incorrectly, but I say again it's "airbrakes."
; ^ )