I have a 2001 Pontiac Grand Am GT with the 3.4L V-6. I changed all four sets of brake pads and had the front rotors turned. I also replaced all four wheel hubs which came with new ABS sensors installed. Now the ABS system seems to come on too early,at about half pedal under hard and light braking. There is considerable vibration in the pedal, which feels like the ABS system engaging but I don't think it should do this under normal light to medium braking. It stops straight, and in a "panic" stop with full pressure applied to the pedal, the ABS does its job with no wheel lock up, etc. The ABS light is not on. I also replaced the parking mechanisms on both rear wheels and adjusted the parking brake shoes as shown in the Haynes manual. I also bled all four wheels.

I read somewhere that the ABS control unit which I believe contains the proportioning valve can be reset with a special scan tool. Has anyone heard of this or had experience with this? I was thinking of asking my local GM dealer also. I also have a 2003 Grand Am and I drove it for comparison, and the brake pedal goes down nice and smooth under normal braking with no vibration felt in the pedal.

This vibration you are feeling, do you know the difference between a pulsation and ABS activation? Sure it'* not pulsation?

This vibration was not there before all the brake work was done?

You turned the front rotors, but not the rear ones?

I have only had the car a short time, I bought it to fix up ad resell. I did not notice any braking issues when I first bought it. It needed some other mechanical work so while I was ordering parts I decided to get brake pads also. I had the front rotors turned by a reputable machine shop that has done work for me before. I did not have the rear rotors turned. The pedal pulsates as though the ABS is coming on but it does it even under light braking at 35 - 40 mph on dry pavement when there should be no wheel lock-up to cause the ABS to react. As I mentioned, I also have 2003 Grand Am and the pedal is much smoother and only goes into ABS mode when you slam on the brakes. Compared to the 2001, the 2003 brakes have a much smoother application. I also tried driving the 2001 with the ABS fuse removed and that'* when I discovered that the rear wheels would lock-up, which is why I installed the parking brake kit and adjusted the parking brake shoes.

According to the Haynes manual you are supposed to bleed or prime the ABS hydraulic modulator, but when I looked at mine, I did not see any bleeder valves. Can someone tell me if it is supposed to have one (or more) and where they would be? They would have to be somewhere near the top in order to push the air out, but I sure did not see any.

I had one person that was interested in the car until he took it for a test drive. He also commented that the brakes were "dragging." I don't think I will be able to sell this car until I get the brakes to operate more smoothly.

As I said before, you must differentiate between, is that vibration a pulsation, or is an actual ABS activation....

Here is what I want you to do.....make a few ABS stops.....you can do this on an off ramp by putting either the left side or right side wheels on dirt/sand on the edge of the pavement, or putting a side on the grass with the other side on the pavement......make a couple stops like this....you will feel the brakes pump, and you will hear a buzz for the ABS modulator.....this will give you an idea of what this particular vehicle feels like with ABS activation.......

Another thing to do, is come off the highway at 65mph, and LIGHTLY HIT THE BRAKES on the pavement......Does the steering wheel shake? Do you feel a vibration in your butt, meaning it is coming from the rear....also do you hear that modulator activating?

Doing this you should be able to determine which situation it is......