Negative is failing via the Mouthpiece.

If a negative test is failing first listen to the closed mouthpiece for leaks. If leaks are heard here then the negative failure is likely due to the 4 mouthpiece inner barrel O-rings. Strip down, clean and rebuild Mouthpiece.

Further Resources

Assembly & Testing Document

Video Link: Stripping Down the Mouthpiece
Video Link: Assembling the Mouthpiece
Video Link: Apeks mouthpiece disassembly
Video Link: Apeks mouthpiece assembly