If the machine is set up properly this is not an issue, the pressure line has a constant purge flow to stop this. This flow is checked on the service and should equate to a pressure of 140cmH2O when checked with the airway pressure line clamped. This is adjusted with PR6.
This should avoid the issue you are having. The only other way this happens is if the circuit is set up incorrectly and the drain line is connected to the airway pressure port.
