2001 brakes

it's happened a couple times, if i'm going over some bumpy ground and use my brakes they go straight to the floor an i end up having to double or triple pump em to get em to work.  another time I had to slam on the brakes to avoid a crash an they went straitght to the floor an i had to double pump my brakes anyone know what's going on.

Does the abs come on at this point? What year is the truck?