You should be able to pull back pretty good, it takes a lot to get air bubbles to come into the syringe compared to if you hit a vein. Sometimes you dont even need to pull back on the plunger if you've hit a vein to know, it will just make its own way up the pin. You'll know if you got a vein when you pull back it would be like pulling water thru the pin, it comes up so quick.