Just to add a few words to those of Steve...
Many hundreds of HDDs have been installed in all types of QB78 family air rifles over the past couple of years. We have had no customer complaints to date that the HDD has failed to work due to extended use. I believe this clearly supports Steve's contention that the service life for this part is, indeed, very long. With all the CO2 saved through its use, the HDD keeps paying for itself many, many times over
Archer Airguns Inc.
Online stores http://www.archerairguns.com