There is a check ball located in the center of the probe in the coupler. Place coupler in a bucket of hot water and then using small brush or tool push the ball towards the top of the probe to free if stuck.
Be certain that the retainer at the top of the probe is in place. Otherwise the check ball will prevent beer from flowing into the beer line. Review the
exploded view of a "D" coupler to assist in identifying parts.
What is your liquid temperature?
Scott Zuhse, Instructor
Micro Matic Dispense Institute