Early in my flight training, while pre flight checking the plane I noticed one of the tires was very low on air. I informed my now ex instructor, who added some air to it and shrugged it off. We flew that day with no issues. As a new student at the time I just put faith in his judgment. I look back on that now and realize that I should have insisted that we cancel the lesson and the plane be checked out properly by maintenance. The tire was low on air and we never determined what caused it to loose air or how fast it was loosing it. Had it lost too much before we landed it would have not been good to say the least. I did learn a lesson though, if I don't feel good about the airworthiness of the plane I am not going.