Car A/C Charging/Manifold Help!

Need some help from the 2cool brain trust. I have a R134a system that I am trying to charge. I pulled a vacuum on it and got all of the old r134 out. However, I am having trouble getting it to charge with the new items. Being new to the manifold gauge set, I am not sure what I may, or may not, be doing wrong. I hooked up the refrigerant can, pierced it, opened the valve, purged the air at the manifold, then opened the blue low pressure valve. The issue is that it is taking a REALLY long time to draw in the coolant. By long time, I mean 30 minutes and it still didn't empty the whole 12oz can. I did notice that the yellow charging hose on the can connection has a valve on it similar to a bicycle tire. Is that what is causing it to be so slow. Is there a trick to it? I don't think there is any other way to connect it and it just seems to me that the can pressure would have to push the pin in but it doesn't appear to be happening or at least not overly fast. The can adaptor that has the piercing pin doesn't have anything that would push that hose pin down either.



What gives???? Please tell me its something easy that I am missing....