A viscous and electrically conducting fluid separates a rotating disk and a parallel non-rotating disk. A magnetic field of intensity B0 is applied perpendicular to the disks. As the two disks are squeezed closer together, increased viscous torque is transmitted to the rotating disk. The non-similar problem is solved using a perturbation in the small squeeze number. The effect of magnetic field on the braking action is discussed for the conditions of the top disk moving with constant velocity or constant force, while the bottom disk rotates with constant angular velocity.