Hyd Fluid Leak Puzzle

[Steve Sherman]

I am baffled by the hydraulic fluid leak that my 91 avant is having.
Seems to work fine for some time (6 mos the first time, 1 mo this last
time), fluid remains full in the res.  Then suddenly, like within a few
miles (10-20), it will lose a bunch of fluid (like the whole res worth
in a short trip), making a nice puddles whenever I park.

Whenever this happens, and I look underneath.  The leak always seems to
be coming from the "bomb" area. The area on the bomb is soaked with
fluid and the crossmember under the bomb is wet there too.  I clean
things up and dry it off, and then run it, and no obvious leaks.  I even
checked every nite for almost 10 days but no sign of oil around the bomb
or any of the lines leading to it.

I have checked for other possible sources, but the res, pump and various
other things that might be leaking are dry and show no signs of
leaking; which whould be especially obvious when it dumps  .5 l or more
in a short time.

I am baffled as to what this might be.  Is there an overpressure valve
on the bomb that might be cutting out every so often?  Any other ideas;
anyone on the forum seen this behavior before?

TIA