Mk4 Rick

I took the car for a pre mot yesterday my mot tester tested the brakes and when he tested the rear passenger one the brake pedal vibrated i felt it for myself.He reckons it might be abs pump or air in the fluid.There is no lights on the dash at all.When i drive the car normally its fine you dont feel a thing does anyone have any ideas what it is?this is on a 2001 mk3 v6