Possible Clues:

• Pop your pop might have liked
• Multiflavor soda brand
• Twenties pop (radar's fave)
• Brand once advertised as 'Tops in taste'
• Radar's soft drink
• Radar input on ''M*A*S*H''?
• Radar selection
• It was once advertised as 'Your favorite drink in your favorite flavor'
• Radar's favorite drink on 'M*A*S*H'
• Venerable soda brand