I must respond. I am currently flying two models without a pressure line connected. One is powered by an O.S.Max .10 and the other a K&B Sportsman 20. They are old models with metal tanks with a vent only and I can do all manner of aerobatics except prolonged inverted flight without a burp (they don’t have an inverted pickup). I wasn’t referring to control line when I said we flew many years without a pressure line. I was referring to early R/C (not just rudder only but reed multi-channel also) and we did a full program of aerobatics with no muffler or pressure line. I recognize the advantages of pressurizing the tank , especially with some modern engines, and use it in most instances, but depending on the engine used, it is not really essential in a trainer.