1991 F350 460 power steering just stopped working
 

I was in a field last night hooking-up my trailer and as I started out, running through tall, heavy grass the power steering just stopped working. I felt it hard to steering and then it may have come back for a second and then it was gone...all in the matter of 10 seconds...gone completely. Well, I'm thinking I've slung a belt or something...I pull over and look at everything and nothing stands out as an issue. I get the truck home and get a flashlight and go over the system, nothing. The fluid is full, the lines are still attached, the pump is making no noise, etc. Typically a powersteering pump makes a little noise when you try to turn the wheels (even when new) but I get no noise at all from this one now.

Any ideas? I'm guessing it has to be the pump, which I've priced and I can buy with-out the res for like $25 cheaper than with it...how hard is the res to swap? Any ideas on how to flush the system when changing? I noticed, when I bought the truck like 2 months ago that there seemed to be "gunk" in the ps res...I'm thinking that someone may have put the wrong ps fluid in it at some point causing this. What fluid is used in this ps system?

Thanks in advance!