Determining what size hose is required to route to the cooler and the fan clutch solenoid is confusing based on the parts manual.
For example, the fitting from the return line from the cooler to the fan clutch solenoid (Fig. 178, Item 1) 4730-00-789-0951  is designated as manufacturer p/n 110-03604 which is apparently an Eaton Aeroquip 90 degree brass fitting of 1/4 NPT x 5/16 hose barb.
However, the hose specified to go over the barb (Fig. 178, Item 7) is P/N 6490610019 which translates into AM General 5741209, which is assigned NSN 4720-01-186-2358  and is also known as GM 6210M.
Per General Motors Corporation, the 6210M specification covers “…a 9.5 mm inside diameter hose made from textile reinforcement and synthetic rubber for use in hydraulic steering applications as a flexible connection between the power steering gear and the power steering pump or pump reservoir.”
The closest fraction to 9.5 mm is 3/8, which is exactly 9.5250 mm. Whereas 5/16 = 7.9375 mm. We checked the dimensions of other fittings called out for use with this hose (e.g., 4730-01-481-6278  ). Its dimension is stated as “NOMINAL INSIDE DIAMETER HOSE ACCOMMODATED 0.290 INCHES 2ND END” or, slightly less than 5/16″.
We further found in the parts manual other references to P/N
6490610019, which gave an alternate AM General part number of 12338330, which is again defined as flexible hose (along with a suffix -X number) of 3/8″ diameter with applications ranging from Air Horn to CTIS (-7), transmission (-1. -2, -5), and power steering (-6, -9).
It seems axiomatic to use a 3/8″ hose with a 3/8″ barb fitting, not a 5/16″ barb fitting as the call out indicates. However, throughout the parts manual it systematically calls out for 5/16″ barb fittings.
We are of the opinion that using a 5/16″ barb with a 3/8″ hose is an error. The barb itself is 1.5875 mm smaller diameter than the hose, and that means that the hose would have to be tightened considerably just to contact the shank of the barb.
We inspected the old and damaged hose removed from the 1038. There were no markings, but it appears to be a 3/8″ hose. Further, 3/8″ power steering hose is considerably more readily available commercially (and at considerably lower cost) than the 5/16″ version.
We ordered and have on hand Gates 350010 Power Steering Return Hose (which is a S.A.E. J189 Power Steering Return Hose
3/8″ x 25′ bulk package).
We can find no valid reason for the mismatch between the sizes of the fittings and the hose called out in the parts manual.
We will update or change our findings regarding the discrepancy between the parts manual call out of 5/16″ barb fittings and the use of 3/8″ hose as we complete the hydraulic plumbing of the 1038.