Brake pedal is pulsating at slow speeds?

I just dealt with this identical problem with a friend's Silverado. I fixed it by removing the front wheel speed sensors and cleaning the mounting surface where the sensor meets the hub. Corrosion builds up on the surface and changes the air gap of the hall-effect sensor. Unfortunately, if you live in a "rust belt" state, this may become periodic maintenance. On extreme cases, I've read that the corrosion contaminates the grease in the hub and collects between the teeth of the tone ring. Cleaning is difficult because you don't want to wash the grease out. The alternative is to replace the hubs - this is only necessary in extreme cases; it's also costly in terms of both parts and labor. Here's an informative video that will help you fix it.

https://youtu.be/ZgiPRG6jffc

Please run down this guide and report back.

1) Safely raise and support the vehicle.
2) Disconnect the front wheel speed sensors from their harness and frame.
3) With a digital volt-meter attached to the terminals of each wheel speed sensor connector, rotate the wheel in a clockwise direction approximately one revolution per second. The minimum reading on the meter should be at least 350 ACmV’s. If the reading is less than that, remove the sensor.
4) Plug the sensor bore to prevent debris from falling into the hub.
5) With a wire brush, sandpaper, or another suitable cleaning device, thoroughly clean the speed sensor mounting surface.
6) Examine the sensor head for warpage or distortion due to corrosion build-up. Check the mounting surface of the sensor head for flatness at three or more positions.
7) GM recommends applying two thin coats of rust penetrating lubricant (GM P/N 89022217) to the sensor mounting surface on the bearing hub and allow to dry for 3-5 minutes. Once it has dried, apply a thin layer of bearing grease (GM P/N 01051344) to the hub surface and sensor o-ring, prior to sensor installation.
8) Install the original sensor or replacement sensor in the hub and make certain it is seated flush against the hub.
9) Re-check the sensor voltage while rotating the wheel as previously described. The voltage should read a minimum of 350 ACmV’s.

Here's how to do the job get the parts from Amazon or Rockauto.

https://youtu.be/cFwjBwhpZ4o

Please run down this guide and report back.

Sounds like bad rotors again. Also check to make sure your calipers are not sticking.

Moderator reinforcement:

I totally agree with Tim-to prove it: Mike it or use a dial gauge and check for out of roundness and the parallelism of the rotor-Caliper piston might not be retracting due to its seal or its binding inside the bore.

Where do you feel the shake? In the seat or the wheel? If fronts are warped you will feel the pedal bounce and maybe the steering wheel shake, if you feel the truck shaking and it seems to come from the seat area, check the rears, get the truck up to speed and use the parking brake, feel anything? Before you do this get under it and pull on the cables to make sure they work, then as you do the test keep the release lever pulled, or if it's a locking pedal apply the pressure so it releases as you do it! Start with light pressure and continue the force unless you can feel it right away.

You need new rotors is my opinion.