Hydraulic problem on 7720 combine

[grefer]

I purchased a 1984 JD 7720.I am having a problem with the header,unloading auger,and variable speed drive hydraulics.They all work fine until the combine has been working for approximately 15-20 minutes,then the header will hardly lift,the unloading auger is slow to swing out and the variable speed on the drive is slow to react.The dealer who I purchased this combine from is stumped and said I will need to bring it back to be tested but I am harvesting and don't want to be down at this time.Both valve bodies have been checked,the pump was replaced and the power steering pump was isolated and determined not to be the problem.If the combine sits for a few minutes idol the hydraulics start to work again,This would seem to indicate that the system is getting too hot,or maybe pressure is building in some area and by passing oil.This combine has a power torque drive and so no oil cooler.Sure would appreciate any comments or someone who has had a similar problem.

[loops]

Just a shot, i'm thinking maybe on the suction side of the pump, a restriction or plugged hyd. filter. If it works at idle when flow is low and can't keep up at high idle with high flow rate maybe it just isn't getting the oil volume needed to the pump.

[ALLFARMER]

I bought a 7720 this fall and found my fuel tank contains enough straw to plug off the filter & 3/8" line too it!
I think you have a hydraulic system sceen plugging off your flow. Hydraulic demand sucking garbage into a screen. I blew one belt this year other than that I am very happy with the machine.