A dip at idle can also be a problem with the brake booster iirc while pressing on the brakes.
When you turn the screw in all the way, does the engine stall? Yes? Good. No? The idle air valve is clogged up and isn't working. Probably corroded.I just got one of these trucks about a week ago and I'm having the same issue. After it's warm the idle dips upon braking. I've all ready tried adjusting the idle screw and it's turned all the way in with no more ability to turn further.
any other suggestions?