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