Question: how to vent ag button?
Answer: Put a 60 mL syringe without the plunger into the G-tube. ... 
After unclamping the G-tube you may hear air or see food or stomach juices rise up in the syringe.
Allow venting for 1-2 minutes.
After all food and stomach juices have gone back down the G-tube, clamp the tube and remove the syringe.