Brakes

There is nothing to be gained by bleeding the system unless there is a reason to think air got in there. Be aware more problems are caused by improper bleeding procedures than are solved. If at any time the brake pedal got pushed all the way to the floor, chances are real good the master cylinder was damaged. That could explain why the pedal feels worse now.

Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons do not normally travel. When the pedal is pushed more than half way to the floor, whether surprised by a sudden leak, or when pedal-bleeding with a helper, the rubber lip seals run over that crud and can be ripped. That usually results in a slowly-sinking pedal, and that often takes two or three days to show up.

This damage can also occur when replacing the front brake pads. The pistons had to be pried back into the calipers to make room for the new, thicker pads. Once installed, the brake pedal has to be pumped to run those pistons back out to adjust them. This is where a lot of inexperienced mechanics push the brake pedal too far and damage the master cylinder. I only use gravity-bleeding to avoid these potential problems. If I do work the brake pedal a little to get things going faster, I always do that myself. I never trust a helper with my brake pedal, regardless how much experience they have.

What I suspect you are describing is the normal buzzing of the anti-lock brake system activating when it is not needed. The clue is the speed you find this occurring. GM's wheel speed sensors develop really wimpy signals to start with, and their signal voltage gets even weaker as wheel speed decreases. By around ten miles per hour, the signals can become too small to be read by the computer. The computer assumes a wheel is locking up, so it starts pulsing that brake intending to get it back up to speed.

The common cause of this false activation is rust build-up under the front wheel speed sensors. The strength of the signal being generated is a function of wheel speed and the distance between the sensor's magnet and the tone ring in the wheel bearing assembly. Rust build-up pushes the sensor further away from the tone ring which makes the speed signal weaker, and the signal gets steadily weaker as wheel speed slows down. Thee fix for this is to remove the sensors, clean off the rust, then reinstall them.