My L3400 HST with 367 hours just developed the same problem. I can answer some of the questions. The operator's manual refers to a hydraulic system "dipstick" but it is really a round sight glass in the left side of the transmission near the clutch lever. Look across the foot rest and you will see it. It can get oily so you might need to wipe it clean. Proper fluid level is at least halfway up in the sight. If the hydraulic reservoir is full, there will be a little bubble remaining at the top of the sight glass. There is one hydraulic reservoir but two pumps on the HST model. The filler for the hydraulic fluid is a screw-in plug behind the seat on the top third of the differential. One pump is inside the transmission; the other is on the right hand side of the engine block as you pictured. As some stated, I would not expect a hydraulic pump the supplies the loader and power steering to fail after only 367 hrs, especially given that fluid and filters were changed at the prescribed interval and the old filters and fluid appeared clean and normal. My HST seems to operate normally but my loader is slow to respond (about a 30 second delay) before any action and then rises slowly with a jerky motion. Power steering is not right either. If I am raising the loader it almost shuts down the power steering. I assumed the hydraulic filter (there are 2) was clogged but knew I just changed the fluid about 100 hours ago. I just changed entire hydraulic fluid (6+ gals) and both hydraulic filters. No issues with the old filters that I could see and the old fluid looked like new. I read all the threads and they are very helpful. My dealer said it sounded like the hydraulic pump and suggested removing it and checking for scoring inside. The pump is apparently not re-buildable and a new one costs just shy of $1,000. However, I did notice dampness under the transmission all the way from the engine back that I didn't expect in a fairly new, lightly used tractor. The leaking pump intake line sucking air makes the most sense of anything I have read or heard and would explain the dampness under the tractor as well as the jerky motion of the loader which would result from inadequate fluid supply to the hydraulic pump that supplies the loader and power steering. I am going to check mine to see it I find any cracks or breaks. None were obvious from casual inspection of the mounted line but that doesn't mean there isn't a break. Kubota paints over the lines so the paint could hide a crack or one could be hidden while the line is mounted. Also, there appears to be a gasket on the flange where the line attaches to the pump and that could be faulty too but, after reading these posts, I am suspicious of the pump intake line itself. I will update you when I investigate further.