Magnaclean professional 2 valves advice

[Magnaclean2help]

I have just been given a magnaclean professional2 (secondhand) and before fitting it I watched the youtube video on servicing it that showed it should slip off the valve pipework by pulling the trigger back once the red wheel is screwed fully in. My trigger did nothing and the unit wouldn't pull off easily so I used a little force and it came free but left marks from the o rings on the brass valve tubes.
My question is now still not fitted I can push the canister onto the valve pipes unscrew the trigger and can still pull the canister off the valve pipes like it's not gripping, do the push fit valves in the canister need pressure behind them to grip? is it the o ring need replacing if they can be or something I'm missing.
Really don't want to cut the pipework only to find this unit is faulty.

 

[oz-plumber]

The canister should grip onto the valve pipes.
If it doesn't than there is something wrong and I wouldn't trust it to seal the system under pressure

 

[Gasmk1]

you need to check the orings they may be damaged or moved

 

[oz-plumber]

I would 'hazard' a guess the grab rings have failed.
The o-rings may have perished if having been dry for some time, but they don't hold the canister onto the pipe sleeve

 

[Magnaclean2help]

The canister should grip onto the valve pipes.
If it doesn't than there is something wrong and I wouldn't trust it to seal the system under pressure

Thanks that's what I thought but needed to check.