Professor Poopsnagle's Steam Zeppelin (1985)

Professor Poopsnagle has made a sensational invention that eliminates all air pollution. But before he can present his discovery to the world, he disappears. The children of the "Secret Valley" obtain an encrypted reference to the whereabouts of the scientist and decide to go and look. Help they receive from professor's colleague, Doctor Garcia, and set off in a hot air balloon in the shape of a red bus.