I have a puzzling problem with my 1990 420SEL. Hard braking is perfect, it stops quick and smooth but, when I apply light to moderate pressure there is a vibration as the car passes through 40mph. It is severe,
it is similar to the feeling of passing over the warning groves in the road as you approach a stop sign in rural
areas. It seems to be coming from the front end but I can't be sure. All front end parts have less than 30k miles on them. I have checked the rotors and they appear smooth and true. The brake pads are original equipment and appear to be in good shape. Tires are MXV4's , balanced on the Hunter 9700 and front end alignment was done less than three months ago. This problem seems to have started suddenly about a month ago. This car has 215k miles and I have not done any brake work since I bought it at 150k miles but all parts seem fine and have worked flawlessly till this problem. Please help, Thanks in advance.

Your problem is most likely a wrapped rotor or 2.
You said: '' I have checked the rotors and they appear smooth and true''
Appearance is not good enough to identify the problem. Only a few thousand of a inch is enough to cause vibrations.
Have your brake system checked and measured for warp.
jackd