72 K2500 Power Steering Problem

[54GMC150]

Greetings All,
I recently got my project truck running. Getting near to completion now. It has the original stock 800 Saginaw power steering box. When i started it Saturday(the first time in years since it ran), the steering wheel immediately and forcefully turned left fully. As yet,I’ve been unable to diagnose the problem. I removed the belt so I could test drive it and check various things and happily, everything is fine except for the stated problem. I am aware of the possibility of this happening with other types of power “assisted” steering like the system in a c3 corvette or ford type systems but this one has me stumped. Any ideas or insights would be very much appreciated. Thanks, Rick