On  my  late  4430,     same   issues.    I  was  told to  take  the  end  plugs out of the  steering valve,   and  test the  spool  with   slight pressure  to see how  bound up it  was:    anything  more than heavy thumb pressure is a  problem. 

Sure enough,   it  was  quite  tight.   Like  'thump it with a  hammer'  tight.      
It's about as  simple and  fast a  quick test  for that  component  before all the  tools  have to come out...    

I  have a  replacement  from   Worthington Tractor  waiting  install,   It was on sale last week  for  575.   But  I can't  say   if  those  aftermarket  boxes  are  worth it yet.