Post poppets DO NOT cause leaks when a QD is properly installed and latched. At that point the poppet is wide open anyway - so what's going to "leak"? - and
the QD is relying on the outer post O-ring to seal.
So, three possibilities: the post O-ring is not doing it's job, OR the post isn't sealing on the top of the dip tube flange, OR the dip tube O-ring is missing or defective and the keg is actually leaking from under the post...
[edit: ok, I was a bit late to the party