the valve spool (in the gear) could be binding when you begin to open the valve but turning the wheel it cannot return back to a resting/neutral position which causes it to keep applying fluid pressure.
I'm curious though, does this happen on both left and right? My first thought was, reversed hoses at the gear. But that mistake will usually cause a rapid *thrashing* back and forth of the wheel. Wheels will go lock to lock until you turn the engine off.
Now that I read through your post again it definatly sounds like a binding valve spool assembly in the steering gear.
Never the less... end result? Replace the steering gear
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'71 LWB
350/350T, PS/PB
Special options? Yes, RUST... Need some?
Garrett
Olathe, Kansas