The most likely issue is that the seal in your auto-release cap is failing - recently I replaced the crumbly seal with an O-ring and it works. It's also possible that yours is set to release far too 'early' (meaning at too high a residual pressure), leading it to release at full CO2 pressure. The VintageAirgunsGallery instructions for the auto-release cap (page 9 of link below) and its diagram (page 22) apply. Though the Master triggers and firing valves changed through the years, I've only seen one version of the auto-release cap. Note the instructions tell us to deactivate the release feature when piercing a cartridge (rotate the rear portion until it pops backward). After the gun is charged, push and turn to activate the release feature.
https://forum.vintageairgunsgallery....ual/#post-3763

Don R.