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Hmmm, just thought I would bring this back from the dead as well. I really hate the steering lock-up I am experiencing. I will test the relationship to the brake pedal today also.
What I did...and it seemed to work for my application was to flush and fill the power steering with new fluid. My SD has over 100k and about 90k it starting doing the same as described. I did the flush and fill...and now it works like new. Of course...I have a V10 and you have a 7.3L PSD...so might be different animals when it comes to power steering.
This is not a "normal" condition. I bought my SD used w/ 30k miles on it. While still under the 30 day / 3k mile warranty I noticed the same condition. I took it back to the Toyota dealer I bought the truck at and got the "it's normal" speach. I refused to take that answer, so they agreed to let me take it to a Ford dealer (with the chance that if they said it was "normal" than I would have to pay for the diagnostic). The Ford dealer was very compliant (likely because it was on the other dealer's dime) and after replacing the PS pump, steering box, and repeatedly flushing the system, the problem was fixed. I have heard from other folks on here that flushing the system will solve it, and replacing w/ ATF vice PS fluid will also help. Not sure if those solutions alone will work, but I do know that it's not "normal"; don't let 'em feed ya that line!
Being in service 17yrs (not ford) I've also seen a steering box adjusted incorrectly which causes it to bind and get tight enough to hinder low speed or parked steering. That being said I also have a 00' f350 with 60k which does in more often than I like and I'm not so sure its due to brakes being applied. I have had it happen without brakes on gravel. I have to think its a pump issue. I run a 800 lbs plow on the frt in the winter and notice it less, poss. due to temp. and thickening of fluid (running synthetic). My 02' X has not done it yet at 125k