/// The other posts are correct but vibration on braking is usually warped rotors. Rotors warp from heat. Heat comes with sticking calipers. What brand calipers do you have? Have you already replaced the master cylinder? Did you check the brake fluid if you replaced it.

If you have Girling calipers and master cylinder you chould check for Dot 3 contamination. There is a chemical reaction between American Dot 3 brake fluid and Girling hydraulic rubber. One of the possible results is sticking calipers. This is possible only if you have Girling calipers. Raybestos makes test strips that will indicate if Dot 3 fluid is present. Since they cost $20 the safe course is to rebuild the calipers if the rotors are warped and use only Dot 4 fluid.

There are other cause of sticking calipers. If the rotors are warped rebuild the calipers.