As Gassy said I think you need to reset the poppit and give everything a proper clean and lube inside. To reset your poppit. You can do this by screwing the back cap all the way in tightly (but do not force anything) to make sure that the poppit face is properly sealing. If this stops the leak, but the leak returns once you unscrew the back cap to your proper velocity you should check your poppit face for damage. There should be no lube on the poppit face, or in the hole in the side of the poppit. Only a slight amount of lube should be on the oring.
If your poppit is not the issue, check all of the orings on the bolt guide, poppit, and bolt for visual damage. Replace any damaged orings. Lightly lube all of the orings and reassemble your mini.
Get back to me if it is still leaking.
