Unusual Braking Issue?

[cam.man67]

I've never experienced anything like this before. The Scottsdale is optioned with heavy duty brakes and the vacuum assist looks like it comes off the power steering pump?

Anyways, the problem: at random times, the brakes are intermittently either working perfectly or my foot is all the way on the floor. The pedal travel is uneven, sometimes (maybe 1/2 the time) they grab when i just barely put on any pressure. Other times, not so much. I mean, the truck still stops regardless, but it's a little concerning to have your foot go all the way to the floor at random moments.

What could be causing this? My dad think it's air in the brake lines, but I'm not sure that's it.