The other day I was mowing hay and I lost all hydraulics. The hydraulics would return after a restart. The tractor will run for about 3 minutes before it will run out of pressure. I have drained fluid and checked inlet sceen in the sum and checked main filter. These all check out good. I have also pulled the filter bypass valve and it appears to be good. I checked the pressure in the tank about the hydraulic pump while running and it is about 75 pounds until you cycle the hydraulics. Then the pressure drops to zero and slowly returns until the tractor has run for about 2 to 3 minutes. Then you can restart the tractor and the pressure will return. I have also opened the rear filler and look for spraying fluid and I cannot see any fluid spraying around. I also remove the shifters and looked for oil spraying around. I found very little oil moving. The oil appeared to be oil being slung up from the gears. I am at a loss as to where to check now. Any help would be appreciated.