Any ideas how to de-pressurize Pyro-Chem Extinguisher?

Thanks for the info eesakiwi, Yeah the tanks were pre-pressurised on the dry chem side. "im thinking the CO2 bottle is to overcome tank side pressure to actuate the relief valve to empty the tank.



I was able to get the pressure out of them last night without making a mess of things which i am happy about lol.
I laid the tank on its side and strapped it down to secure it then slowly started to loosen the Pressure gauge on the side of the tank till it started to leak. I left them overnight like that and this morning they are down to 0 pressure and i took the gauges completely out to make sure. I'll be taking the brass top off and cutting the tanks in half