I had to get a new clutch master cylinder and rebuild the hyd. booster at the same time on my Champion grader. I got some help and put everything back together but when I went to lift the moldboard had no hyd's. I checked the reservoir and it didn't register on the dipstick so I figured it had lost too much fluid. I added a pail of oil and got the oil full but still had no hyd's. The steering from a different pump worked. I left the machine idling and looked for an obvious problem and nothing seemed out of order. The pump was turning and it felt like fluid was moving in the hoses. Weird. I got in and revved it up to maybe 1/3 throttle and then the hyd's were working. I let it idle and everything seems good again. Is it common after having some hoses off for about a month that the hyd. pump needs to spin faster to pick up fluid to prime itself?
Very well could be may have had a bit of air in the suction line that the pump couldn't evacuate. A fellow I know had a tractor that unless it was parked on a hill would have the pump pick up oil till you went to the hill for a visit
Hydraulic gear pumps are not the best at suction and benefit with the reservoir placed higher than the pump inlet. Large supply hoses help also. Many gear pump systems incorporate a flow check valve on the pump discharge side to hold fluid and disallow drainback into the reservoir dependent upon design. Reads as if your reservoir has more than one oil supply outlet, and possibly the main supply pump outlet of the reservoir was physically higher than the steering pump outlet; or wear in the main gear pump exacerbating the lack of suction aforementioned. Just for chits and grins, I'd do pressure testing on the system if compromise/wear is suspected. End plate wear is common in gear pumps and although the system may perform acceptably well at high engine rpm, the low speed performance may not be where it should be.
I think it had to do with a couple lines taken off the booster for the clutch master cylinder for a few weeks. Hyd's always worked at idle before that. After they started working, they still worked at idle so Tyler could be right about a little air getting in the suction line.