98' F-150 brakes/anti-lock problem

Thanks everyone for the great advice I read here daily. Now I hope someone can help me.

When I brake at a light or off ramp and hit a pothole or bump in the road, my brake petal goes to the floor and I hear a clicking sound in the dash. I assume it is the anti-lock indicator. If I take my foot off the bake and re-apply I can come to a normal stop, but I have had a few close calls.

I took the truck to Sears 4 months ago for brakes and got rear shoes. They said everyhing else was fine.

Any ideas? I'd like to some talking points for when I take it to the dealership for service.

Thanks again, JR