Since the problem seems to occur only when brakes or steering are called upon.......I will hazard a guess that there may be an issue with the hydraulic pressure accumulator that serves as a fluid pressure reservoir for these 2 hydraulic functions.
I am presuming the 5F models with brakes that operate from the main hydraulic system are similar to the older 3F series I worked on in the 80's.
On the 3F models there was a hydraulic accumulator which stored pressure from the main hydraulic system as an "emergency brake/emergency steering" in the event of engine failure and subsequent loss of hydraulic system pressure.
The old 3F accumulator would store enough pressure/volume for about 3 brake applications and maybe one turn of the steering wheel before it was depleted.
When I return to work next week I will dig in our old manuals to see if anything is available.