I'm with Sam. The clutch slave cylinder lives in a completely different
and harsher environment than the clutch master cylinder does. There is
no reason to expect the part with a cushy life to wear out at the same
rate as the one exposed to engine heat and outside moisture. That said,
I tend to replace them as a set on Vanagons I intend to keep a long time
and travel long distances in, unless I know it has already been
replaced. The slave cylinders fail much more often in my experience.

Mark

Sam Walters wrote:

> Actually, it is the opinion of only some that when one goes the other is
> sure to follow.
>
> As I have posted several times before, I once searched the entire
> archive for instructions on how to do this job and found folks rather
> evenly divided over whether replace one or both.  Most of those who say
> replace both do so on the advice of their mechanics.
>
> No one has anything other than anecdotal evidence.  And every time both
> are changed, it eliminates the possibility that in that van the one that
> was still good would continue to function just fine for several more
> years.
>
> The replace both position is repeated more often, but I don't think that
> this means that the alleged underlying factual assertion,  that both
> fail close to the same time, is actually true.
>
> I have changed out 6 slaves in my years of Vanagon ownership but never a
> master.  Others have reported only having master cylinders fail. ....
>