Do you need an air brake endorsement for a vehicle under 26000 pounds with air brakes in the state of New Hampshire?

There is no actual air brake endorsement - if you test for a Commercial Drivers Licence (CDL) and either fail the written portion, don't road test in a vehicle equipped with air brakes, or both, you get an L restriction, prohibiting you from operating a commercial vehicle with air brakes. The only times you need a CDL for a vehicle or combination under 26,000 lbs. GVWR is if it's either carrying hazardous materials in a quantity sufficient to require placarding, or designed to carry more than 15 persons (including the driver). In the case of the former, you would need at least a Class C CDL with Hazardous Materials endorsement, and air brakes (if the vehicle is so equipped), as well as the K endorsement for a tanker, if the vehicle is so equipped. For the latter, you would need a P endorsement for passengers (S for school bus), as well as air brakes, if the vehicle was so equipped. If the vehicle you're operating doesn't fall into the criteria of requiring a CDL class licence to operate it, you don't need to be tested for air brakes.