Megaflo bubble recharging or expansion vessel

[DursleyDonkey]

I have a Megaflo cylinder that is about 18 years old. How often should the bubble need recharging? It drips into the tundish when it's heating about every 2 months, and recharging the bubble solves this. (I always leave the valve open until the downstairs taps run completely dry.) Does this sound about right, or is it too frequent. If its wrong, what might the fault be?
Also, is there a way of adding a expansion vessel to the system to avoid all this messing about?
Thanks.

 

[muggles]

Your baffle is goosed, an external vessel is the only solution. Needs fitting by a G3 Registered installer, who can also check that all your safety controls are operating correctly