1.
If it has rear disc brakes - Warped discs?
If it has rear drum brakes - Warped drums?
2.
ABS cutting in when it shouldn't?
If you have no ABS light showing on the dash I'd take a look at the rear brake discs. If worn or warped a new pair of discs and a new set of pads should cure it. I drove all the way from the south of France back to the UK with excessive vibration every time I touched the brake pedal. A new set of front discs and pads cured that.
ChicksFan, September 2011
