I looked in to getting my CDL a few years ago, even went to the DMV to get the manual to study. I distinctly recall needing a CDL for air brakes, any weight.
There are some loop holes for farm use, but for general purposes I am pretty sure air brakes require CDL.

On another note, I have been told by many antique big truck owners that say even with air brakes, as long as they have antique tags they don't need a CDL.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-