What gauge syringe are you using? Liquid should not be able to "drip out" of the syringe unless its:
1 - A very large needle
2 - There is a leak in the syringe barrel (it is not a vacuum)
3 - The liquid in the barrel is undergoing some sort of reaction and is actually expanding.
In the case of #3, I have seen needles drip because the liquid inside is heating up and expanding, but that is rare.
Could you elaborate?