Megaflow air bubble recharge advice please

[swayzak]

Hello

We have an old Megaflo cylinder.

We are not sure which one of the twist valves is the air release valve (used for recharging the air bubble).

I have previously used the blue one (nearest to camera), but am now not sure if it may actually be the larger black further back in the attached picture.

Can someone advise please ?

Thanks

 

[muggles]

It's the large black one

 

[swayzak]

Thanks. Oops. What will have been happening when I released the blue one then (sort of gushing, bubbling sound from inside the tank) ?