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

Hey everyone, so I had someone give me some Pyro-Chem PCI-50BC extinguishers "7 of them with a handful of other smaller extinguishers"

I've gathered some information as to how they are activated and how they discharge in normal operating setups.
They use a CO2 cartridge to pressurize the valve which opens the tank to expel the dry chem.

I don't have any of the system just the tanks.

I'm wondering if anyone has any info or tips for a safe way to depresurize these tanks to recover the brass and the tank. they are charged to 350 PSI.

Tank
https://imgur.com/4d0j4G2

Regulator
https://imgur.com/W0I5gF1

I dont know.

But is the powder filled tank pre pressurised?
Or is it pressurised from the built in CO2 gas bottle After you trigger the extinguisher?

If its not 'Pre pressurised' then you should be able to unscrew it and pour the powder out.
But CHECK 100% before trying..

Or maybe turn it upside down & try triggering it, maybe with one thats been used a bit.

I hate the powder in these things, its smell, its taste, its grittyness, the way it gets everywhere once used.
I have seen ( & been the subject, nah, victim of ) somebody saying "hey this is a great idea, lets set this off..."
& Before anybody can say "No" ( "this is not a good thing to do while seriously inebriated, in a RV trailer, full of people")

They do it.

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