"According to the Federal Motor Carrier and Safety Administration, you DO NOT need a CDL with air brake endorsement to drive a vehicle that is under 26,000 GVWR that has air brakes. The reasoning behind this is that since a CDL is not required to drive the vehicle, there is no way to require the "air brake" endorsement under the CDL guidelines.
Also note that your Class C license does not contain an "L" restriction.
References:
FMCSA §383.5 Definitions.
Commercial driver's license (CDL) means a license issued by a State or other jurisdiction, in accordance with the standards contained in 49 CFR Part 383 , to an individual which authorizes the individual to operate a class of a commercial motor vehicle.
Commercial motor vehicle (CMV) means a motor vehicle or combination of motor vehicles used in commerce to transport passengers or property if the motor vehicle-
(a) Has a gross combination weight rating of 11,794 kilograms or more (26,001 pounds or more) inclusive of a towed unit(s) with a gross vehicle weight rating of more than 4,536 kilograms (10,000 pounds); or
(b) Has a gross vehicle weight rating of 11,794 or more kilograms (26,001 pounds or more); or
(c) Is designed to transport 16 or more passengers, including the driver; or
(d) Is of any size and is used in the transportation of hazardous materials as defined in this section.
FMCSA §383.95 Air brake restrictions.
(a) If an applicant either fails the air brake component of the knowledge test, or performs the skills test in a vehicle not equipped with air brakes, the State shall indicate on the CDL, if issued, that the person is restricted from operating a CMV equipped with air brakes.
(b) For the purposes of the skills test and the restriction, air brakes shall include any braking system operating fully or partially on the air brake principle.