GMC Troubleshooting Questions? Ask a Mechanic for Answers
Sounds like you might have two separate issues. Where do you feel the vibration at idle?
Does it sound like a humming noise? Do you have any warning light that come on along with vibration?
Ok lets start by trying to eliminate the abs system. In the fuse box under the hood pull the fuses that are listed as abs. Then see if your vibration at brake pedal is gone.
Yes it will be fuse #9 and fuse #61. Both are for the abs. Doing this might make your abs light turn on but your brakes will still work except the abs.
those will go off once you put the fuses back in. Did the vibration go away at the brake pedal?
Ok after installing the fuses turn the vehicle on then off. then on again and the brake lights should be off.
Know that we ruled out the abs system lets check some things on the steering system. Check the fluid level of the power steering fluid. this reservoir will be on the drivers side usually close to the power steering pump.
This systems brake system has a hydro boost for the brakes. The power steering pump supplies pressure to the hydro boost for your brakes so if the pump is failing it would make sense to feel a pulsation at the brake pedal.
It should be up to the top line with the engine off. This level doesn't really change much with engine hot or cold.
Once you top off the fluid level check to see if the pulsation is still there.
If the pulsation is still there then I would recommend that you replace the power steering pump.
Turn the vehicle on then put your foot on the brake and start to turn the steering wheel. If the pulsation gets worse while you turn the steering wheel then that's another sign you have a weak power steering pump.
It still seems like a power steering pump issue. By what you described at low speed driving also points to the pump. At higher speeds it would not be noticeable because there is not much load on the pump.