Can you use your employer 26 foot box truck with air brakes to take the CDl road test?

Air brakes are not a factor in determining whether or not a vehicle requires a CDL. As for your employer's box truck, the length doesn't matter - the weight rating does. If it has a weight rating of 26,000 lbs. or less, it won't be useful in helping you get a CDL. If it's rated at more than 26,000 lbs. GVW, it can only be used to get a Class B CDL, unless you're attaching a trailer of more than 10,000 lbs. GVW to it, in which case, you can get a Class A, but you'll have a "no tractor-trailers" restriction.