Can you drive an air brake vehicle that is under 26000 lb gross vehicle weight with an ordinary lincense?

So long as the weight rating of the vehicle doesn't exceed 26k, yes, with two exceptions - you would need a Class C (minimum) CDL with hazmat endorsement if the vehicle is transporting a quantity of hazardous materials which requires placards to be displayed, and you would need a Class C (minimum) CDL with passenger endorsement if it is a bus designed to transport more than 15 passengers (including the driver).