steering is hard to turn when trying to park, but not all the time. it will go on 2001 Ford F-250 Super Duty

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
easy at first but if i need to correct it a little et is very difficult to turn
(1) Answer
seem to get more difficult with brakes applied? if so, steering pump and/or hydroboost unit may be getting weak.