I have the same problem with my white planter.   I have the fan plugged into remote #1.

I slow the flow on #1 as slow as I can and still maintain air pressure - doesn't take much.  This allows remotes 2 and 3 to work properly.

I also do this on the sprayer with hyd. pump. Tricky to get just enough flow to pump and still allow boom to fold,

edit: Forgot to add that tractor is '89 7120