My brakes have done this symptom for a while now and i have had it to two different mechanics. While pulling up to the end of a driveway, parking lot, stop light, etc..... My brakes will make this "cow mooing type sound" and i feel bad hesitation in the brake pedal. I have to use force on the pedal to stop. This gets worse and worse until over time the red brake warning light comes on and stays on and it feels like from that point on i have only manual brakes. Both mechanics bled the brake lines and it was good for a coyple of days and then right back to the mooing. When i brought it back to the last guy , he replaced the master cylinder and bled the lines again and it was good for a couple of days. Now back to the same old thing........any advice or solutions will help......this is very frustrating and not safe. Thanks

 

Sounds like a bad brake boost

 

If/when the master cylinder is replaced, the master must be bench bled first, then gravity bled at each wheel. The automated bleed must be performed on the ABS unit, then bleed each wheel. This will not do anything for the noise, but it must be done when the master is replaced.

After the system is properly bled, if the noise occurs again, see if this is similar to the problem you're experiencing: http://ww2.justanswer.com/uploads/GM...s_bulletin.pdf It does not specifically mention your vehicle, but the diagnosis and repair procedure is the same.