I have a 1968 cougar xr-7 that I am restoring for my first car. I installed a new booster and master cylinder and I am still having issues braking. When I apply pressure on the pedal I feel the power brakes work and the car begins to stop easily. Then the pedal goes a little more than half way down and it releases the power brakes and I have to push very hard to make the car stop. It's almost like it goes back to manual brakes. I don't know what I should do. Any advice?