The gradual deteriation of a sphere is down to molecular diffusion
This diffusion takes place place as the nitrogen gas migrates through
the molecular structure of the rubber diaphraghm. Much the same as a
helium balloon going down, not because it's got a hole in it, just
because they do!!
When you buy a helium balloon for the kids, you expect it to float up
rapid, the next day or two we expect it to be on the floor, but still
with no holes in it. If you had purchased a foil type balloon, this
would stay up for much longer, the reason being that the molecular
structure of foil is closer than rubber.
All spheres go down, we cannot avoid it, it's the way things are.
Typically on the Citroen the spheres reach about half the pressure
within 3 years, if you have them recharged at this point there will be
no damage to the diaphragm and they will last for another 3 years or
If you leave it untill the suspension is solid, the diaphragm will hit
the top and at this point irreversable damage occurs, therefore your
sphere is scrap metal.
The shelf life of a sphere is years and years, as when not being used
the gas has to migrate not only through the rubber diaphragm, but also
through the steel wall of the sphere, this takes a lot longer as the
molocular structure of steel is a lot better.
When the diffusion takes place (ongoing from day one) the gas does go
into the LHM system, but this is so gradual it will work it's way out
to the reservoir and you wont even notice it.
I am suprised however that you managed to get a warranty claim to
stick!! Most warranty companys will only pay out on sudden failure, not
normal wear and tear. Lucky you !!!
Sorry its a bit long winded, but I am not to good at explaining