Power Steering Fluid boils over - steering works fine... How???

Hi... I would like to take my son to the "All British Car Day"
this summer in our 1964 Jag but the steering fluid reservior
"boiled" over the other day as I was coming home from work with it.

The best guess I have heard is a stuck relief valve... Is that a
real possibilty? The steering works fine under all conditions, no
noises etc, no extra effort just the fluid heats up afer about
10-15Km... The "problem" seems worse if I don't exercise the steering
[on steady, 4-lane hwy driving].... Does the "relief" valve simply
provide fluid circulation when no demand is being made to steer the
car? I am only suspecting that the + displacment pump must keep fluid
flowing constantly and hence the need to bleed-over at a higher
pressure until needed by the box.... Would that valve
likely be on the pump on intergral to the steering box?

Or is this pure speculation without regard for reality?

Any/all comments on this are greatly appreciated!!!
Roy