I had my brake pads and discs replaced yesterday but this buckling effect still occurs. It only happened a coupld of times pre brake fitting but it did it again last night. When i slow down to almost a stop it occaisonally feels like a buckle of the wheel/brake and it makes a slight chunking noise.
Sorry best way i could explain
In the US, many autos have discbrakes only on the front wheels. Do you have 4 wheel disc brakes or are the rear ones equipped with brake shoes? If so, perhaps the sound and effect are coming from the rear wheel(s)...
Brake pads and discs do need quite a few miles to bed in properly, I used to work for Ferodo (Brakes) and they have a great technical line, i'd give them a try 01274 723481