Can only answer part of your question, whcih is why does the pressure rise. I have expereinced this phenomenon in flight going from the cooler southern climate to the hot northern climate. i have observed the high pressure guage rise in stages with no pum blip. Thus I conclude it must be thermalk expansion of hydraulic fluids.

Gerard, et al,

 

Note that the cylinders (and their fluid) located in the nose wheel well are subject to heating by cowl heated "cooling" air leaking into the well.