I recently pulled the coolant pressure tank on my '89 2.1l WBX and
found three significant stress cracks that looked ready to
fail.  Photo of one here:
https://picasaweb.google.com/dbeierl/Vanagon1989PressureBottleStressCracking#
- it looks worse in person than in that still-installed flash picture.

I would suggest draining and removing the tank and examining it with
a strong light shining through it from behind and above.  The
likeliest areas for stress cracks are at the areas immediately above
and below the joint running around the circumference; the third crack
on mine was on the neck, in line between the pressure cap and
low-coolant sensor.  The first two (one above, one below) were long
and clearly formed by repeated flexing of the tank.  I estimate that
patching them on the road would be very difficult except by plastic
welding, because of the proximity of the joint.  The third was much
shorter, gaped open in the middle and was of a different character
generally.  If I learn anything of significance about it I'll say so.

Yours,
David