The post o-rings are likely too small. Many people (including companies selling kegs) mistakenly believe that they use the same size o-ring as ball locks, when they actually require slightly larger. The issue you describe is exactly what happens with a ball lock o-ring on a pin lock post. I'm on my phone, but if you search here you should find the size you need.
I'm trying to find some porn filmed by a dog wearing a GoPro. - SharonaZamboni