I have an MPV-50 engine monitor that includes continuous
reading of the hydraulic system pressure on my Lancair IV/
I have seen the same thing, particularly on a flight that started
on a cool morning departure from my home drome and took us to the desert
for a 110+F day. On descent, pressure rose from its customary 1250
to 1500 psi at which point Bitching Betty started to complain so I
bumped my speed brakes which are hydraulic, and lowered pressure
back down to my customary range. Same thing sometimes
happens in reverse, a hot soak on the ground, then a climb to 10-11,000
feet for a cool soak, and at some point the hydraulic pump kicks on
to restore pressure to the desired level. My TR-182 used to the
the same thing routinely when departing Phoenix in the summertime.
It is a real effect, and makes one wonder about very high pressure
events arising from FL 270 (say) cold soak terminating in Phoenix on a
July afternoon. Hmmmm....
Fred