2006 Express 2500hd van hard to turn

My power steering was hard to turn intermittently.  It
started after a small leak at the gear box developed one
cold morning. The leak stopped after it warmed up. I refilled
the fluid. I did the normal air bleed procedure, back and
forth from stop to stop and it got better only to reappear
the next morning. What I did next was wrong.  I changed
the steering gear box out.  Added new fluid. Only to have
the problem reoccur.  I searched the web and thought I'd
found the solution, traped air. One video instructed that it
needed to be vacuum bled.  I searched for a adaptor for the
reservoir.  I'm sure they are out there but which one? So I
dug for more info and discovered that I had a hydro boost
system. The power steering pump supplies hydraulic
pressure to both the steering gear box and brake system as
well. My van is diesel.  It is on other 2500 and 3500
vehicles as well. And the bleeding procedure is different
than the vacuum boost.  You raise the front wheels off the
ground,  depress the brake pedal and then turn the steering
wheel from stop to stop about 20 times. Don't hold it on the
stops. This removed the air from the system and the power
steering is now smooth.