My car has brake problem

My car has brake problem. Some times on non paved roadswith a speed of 2mph the brake pedal kick back with a vibration to the pedal and a sound comes. The same things happens when you drive on ashphalt roads and a sudden brake application. Is this any relation with ABS or sensor. I changed the master cylinder and all the discs, front and rear but no change. I even adjusted the wheen disc in a lathe since the mechanic told that it has got bend. But when we ump the brake there will not be any problem but sometimes the brake slips irrespective of the speed. Please help me