I have a 1999 F150, I recently am having problems with my breaking system. They are soft now, I have not had any work done to my truck that would involve the break system, but now If I press hard on the break they will work, but if I press slowly or pump them the pedel bottoms out. I have not found any leaks on the system. I am worried that it my be the master cylinder seal, but I wanted some thoughts before I started on that. Any thoughts?