If we assume for a moment that the problem is caused by excessive pressure
in the system when the temperature has risen, then one possible solution
is adding a small accumulator. It should have a pre-load slightly above
normal operating pressure, say 1000 psi.

If the fluid expands, it will push the excess volume into the accumulator
instead of drastically increasing the pressure.

e.g. http://www.flexicraft.com/hydrop2.asp?catid=6&subcatid=358
part number MHY161000. I'm sure Parker, McMaster, and others
would have an equivalent or better part to suit this need.

-bob

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