I have a 1995 Truck,7.3L, used mainly for work & pulling trailers, no mods,brake pedal has been low but working normally. Then the pedal started to react normal on first push, then hard (like a booster problem) on the second push. Replaced the Master Cylinder, still have same problem, could this be a vaccum pump problem?

 

yeah it could be.pull the belt off and turn the pump. it should be loose then get tight and break over to loose again.