Is it maybe a CPT50S?

This might help. http://www.bas.uk.net/docs/Mk4_Vario.pdf

This says it will go full scale and stay there up to 3-5 minutes on initial powerup. Have you waited that long to see if it will come back to zero? Worth a try.

More manuals here. http://www.bas.uk.net/manuals.html

Best of luck with the vario!

Steve Leonard