Brake booster?

My wife has the truck listed above, she says the brake pedal goes down like sounds to me there would be air in the line. I've bled the brakes, I never experienced that when driving it but I did experience the brakes worked fine except one time it was very hard pedal, didn't think I was going to stop getting off a highway ramp. It stopped but I had to push really hard to get it to. Since that one time, no issues when I'm driving but she says she still has the same issues with pedal going down sometimes. Is there something that would cause both symptoms, booster or master cylinder? I've worked on cars for over 40 years for myself and never had issue with both at once. Any suggestions would be appreciated.
Thank you,
Terry S.