• Depressurize: Never attempt to remove the plenum or end cap without ensuring the rifle is devoid of air.
• Lubrication: Use a small amount of silicone grease on internal O-rings (like those around the valve seat) during reassembly.
• Tools: Replacing the valve O-ring can be challenging; you may need tweezers, cotton buds, and specialized tools (pin tool or snap ring pliers) for the end cap

https://huma-air.com/fitting-instructions/