1997 Toyota Land Cruiser Question: My brakes are real mushy when I press them; while driving they become hard.

I am having a problem when I press my brake pad. There are times when they are mushy (When I haven't driven for a hour or so) If I experience frequent stops, then they may become hard at the next stopping point and all I have to do is barely touch my brakes and they will stop the truck. I am noticing a vibration in my steering wheel when my brakes are hard as if the calipers are not retracting like they should. During the times when my brakes feel hard my truck seems as though my brakes are engaged when I am driving and it appears to want to stop the truck. I sometimes smell a burning odor and see smoke coming from my front passenger brakes. The smoke doesn't occur daily but everything else is a daily occurrence.