There should not be any significant amount of solvent in the vent flask. If solvent is dripping into the flask during the extraction, there may be a leak in valve Q7 and it may need to be cleaned or replaced. See XT Service Procedure #5. Another possibility is the leaking is coming from the safety valve and the valve will need to be replaced. To determine if the safety valve is leaking, see XT15 Service Procedure #16, Point #4. If solvent is dripping into the flask at the end of the run, the water was not turned on or the water was too warm to cool the condenser.
Too much solvent dripping into the vent flask will result in an "ETS Heater Overheat" on the instrument's computer screen.