most hydro outlets wil do it.
Tell them they are diaphram FCH accs, they will tell you " you need new ones mate, the diaphram musy be burst", to which you reply
"No, it's just that I have let all the pressure out"
Used to be about £15 for a recharge.
[no harm in getting them filled to max as a starting point so you can be sure what the problem is......]