It is very easy to ruin the hydraulic pump due to the way the suction run is designed, The reservoir is mounted low and they do not gravity feed to the pump, I have worked on dozens of them over my career, When I do that process I always put a 1/4 flat face coupler on the pressure test port with a hose into a 5 gal bucket before starting the machine, that allows air in the suction run to be pulled out. the pump does not have to pump air against static pressure in the system I have seen pumps destroyed during the servicing process. I personally do not see how pressurizing the reservoir is of any benefit as common sense tells you that you are also pressurizing the return side as well and as I said, I have been there many times in my 50 yr Case career and freeing up the flow on the output of the pump is the quickest safest way.
Also just my two cents and do not want to start a war on oils but I would use nothing but Hy Tran Ultra in any hydraulic system I cared about. Good oils have additives to help the oil from foaming and I have seen first hand the results of inferior oils, also there is a difference in cylinder packing life with HTU. I am convinced of that. I was threatened by a Oil company for discouraging my customers from using a certain brand. I have posted here before on this problem, this certain brand oil swelled the piston packing material to the point that the cylinders would literally squeal when moving and when rebuilding they had to be pulled apart on our rollback with the winch. In service I was on the front lines fighting these problems and I believe stand on pretty firm footing. I do not know of any other oil with the exception oh HY Gard JD oil that I would put in a machine that has to do 8-10 hrs a day. There may be other oils out there, probably are, but I would just as soon stay with what I know works.