what are people's theories on these? Some people I know say these have to be replaced as a matched pair, i.e. if one side is leaking then you still need to replace both sides.
I've never done that, I've always only replaced the side that's leaking. It's obvious to me that things like pads, discs, drums & shoes have to be done as pairs but doing wheel cylinders doesn't seem to make much sense, unless it's that the other one will still be slightly worn and not have the same effort as a new one?