Loss of power assist steering on J.D. 400

[WingDoctor]

I'm having a problem with the steering on my old 400. The power assist steering has quit working suddenly. Checked all of the obvious and see nothing out of order. The one thing I did find was that if I use any of the other hydro controls (ie lift the mower deck) while steering, it steers just fine. But during normal cutting, it steers like a tank. This tells me that I have a problem in the hydro system, but I'm not sure where to start. Any thoughts?

 

[beenthere]

:welcome:

A search on TBN may bring up some helpful ideas where to start.
One thread recently comes up.
http://www.tractorbynet.com/forums/john-deere-lawn-garden/220028-jd-400-power-steering.html

Surely there are others that can help.