helo,
i dont think that what ever u r saying is right u have mis understood the concept,let me give it a try..
at the end if the expiration the intra pleural pressure is more negative that the intra thoracic pressure coz of the elastic recoil that pulls the alveoli inwards, at the beginning of the inspiration when the inspiratory muscles contract they further reduce the intra pleural pressure and the intra thoracic pressure causing the air to move in from the outside, as the inspiration continues both the intra pleural and the intra thoracic pressure begins to rise such that at the end of the inspiration both the intra pleural and the intra thoracic pressure are equal.
now regarding the drainage its due to the pull of gravity but the drainage can definitely be aided by using suction,or asking the patient to do any form of exercise like marching,bicycle ergometer,static walking.....and so on ,it can further be assisted by asking the patient to do bouts of coughing followed by breathing control / diaphramatic breathing.
hope this will be helpful.
regards