I recently realized that when the screw retracts it is pulling in air after a few seconds. What seems to happen is when there is melt in the nozzle it seals it off and the screw retracts normally filling with resin, but after a few seconds the melt in the nozzle drops away and with the nozzle open the cylinder pulls air through it. This explains why I've always needed very fast screw speeds. If the screw speed is not very fast the cylinder fills with more air each time the screw retracts and eventually the shot will be almost nothing. Is there some mechanism to stop air from flowing in that might have failed? Or am I missing something about how this process is supposed to work?