Ford 2120 - Hydraulic Oil Flush & Replace - help please

I am the new owner of a used 1990 Ford 2120 tractor, 4x4, w/HSS 4x12 transmission and loader. Its the first tractor I've owned/maintained, but I am very mechanically inclined and intend to do most maintenance and repairs myself. Bought the tractor with about 850 hours and have put another 50 or so hours on it before my first breakdown.
It is leaking hydraulic fluid from the solid pipe that runs from under the hydraulic pump back to a valve for the loader controls. That hyd pipe is part#: SBA-340702120. Can't find it new, now searching salvage yards. If no luck, I will have to repair the pipe somehow. The hydraulic fluid that leaks is a milky butternut color. So there is water in the system and I need to flush and replace all the fluid.
I've purchased a Ford 2120 service manual, but haven't received it yet, so I'm flying blind at the moment.

What I've learned so far from reading this forum and my other research and my current plan:
0. Replace/repair leaking pipe
1. Run the tractor for a while to warm/mix the oil.
2. Drain oil, remove filter and empty oil from the old filter.
3. Put old/empty filter back on.
4. Fill with new hydraulic oil (how much? mix oil with diesel fuel?)
5. Run tractor, cycle all hydraulic cylinders (loader and 3pt lift).
6. Repeat steps 2-5, 1 or 2 more times, with the hope that after each change the oil become more clear
7. Put new oil filter on and fill with Ford/New Holland 134 Hydraulic Oil.

My questions:
What is best approach to flushing and replacing the hydraulic fluid?
Comments or suggestions on my plan above?
Other Tips and tricks for the do-it-your selfer, so I don't end up with oil all over my garage floor?
How do I prevent air from entering the system or must I bleed the air after the final fill of oil? If I must bleed the air, how is that done?

Thank you,
Scott

4. Fill with new hydraulic oil (how much?

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I don't know the capacity, but that would be in the Operator's/Owner's Manual, not the Service Manual. Routine maintenance items like draining and refilling fluids and replacing filters are all in the Operator's/Owner's manual. The Service Manual assumes that you have access to the Operator's/Owner's manual and so it does not repeat that information.

mix oil with diesel fuel?)

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Diesel fuel is just a fairly lightweight oil without much else except maybe detergents for the fuel system, which aren't going to do much in the hydraulic system. I would mix some ATF or acetone with the fluid for the first flush, but that is all.

6. Repeat steps 2-5, 1 or 2 more times, with the hope that after each change the oil become more clear

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Hopefully it should be fairly cleaned out after the first flush with the ATF or acetone, and one more flush should get the remainder of the ATF or acetone out of the system. The third fill-up should be the final one you should need, so I would drain it after the second flush and then repair or replace the line that is leaking before filling it that third time.

Where exactly is the line leaking, at one of the connections at either end, or somewhere in the middle?

thanks for your reply Sean.
When I asked about the volume of oil to use...I know the mfg recommended fill volume (8.75 gallons), I was asking if I can get away with not filling to full capacity for each flush to save some oil/money.
The high pressure tube is leaking in the middle of the tube, where it was covered by a rubber block & bracket all its life. Why do you suggest I wait to replace the tube until after the second flush??? I planned to replace first, to prevent oil getting everywhere when I warm/exercise all the oil during each flush cycle.
I ve attached some pics of where the leak is occurring. I can t remove it from the rubber block/bracket until I disconnect at least one end of the pipe, and I only intend to do that once.
Thanks again,
Scott

I recommended replacing the line after the second flush to keep the new line from getting the contaminated oil in it. The way I read your original post, I was thinking that it was leaking but that you had been using the tractor that way so it wasn't too serious of a leak where you would lose too much oil doing the first two flushes. If it is leaking so badly that you would lose too much oil doing the first 2 flushes, then by all means go ahead and replace the line first.