This is really to Frank C.
But I posted it instead of E-mail because others may have similar problems.

Frank,
If it is, as you say, sporadic, sometimes at full power, sometimes not,
I would not suspect the pump.
I know very little about Vanagon Power steering but I am a hydraulicist.
It sounds to me like there is crap in your system relief valve causing 
it to stick open for slightly longer than it should.  I have no idea
where it is on a vanagon and it may be integral in the pump and not
separately replaceable.  I wish I knew more about the system in the
van so I could help you more, but you may want to try to locate it.
The best way to check for leaks would be to crank the wheel all the way
one way and have someone hold it or bungee cord it so the system is at max
pressure. (don't do this for more than 6-7 minutes, it'll get really hot.)
and crawl around looking everywhere for a leak.
Since it occurs when turning both ways I expect that the problem is 
before the actuators (that's why I pointed at the System relief) but
you never know.

Oh one other thing, is the belt slipping?  Is it good and tight?
Does it squeal with the wheel fully turned either direction?

Once again sorry for my ignorance of Vanagon power steering, but I
prefer the tight turning radius of non-power steering.

Cheers

<sstones@io.org>
SStones             Toronto,  Ontario.