Intermittent Braking Force...

[Niall]

It's more likely a shadow on the disc from the pads being stood while warm - you'll probably see a faint transparent print of the brake pads on there somewhere where they've transferred some pad material across. That'll give a touch more braking power at one spot without actually giving you vibrations as such. Just makes the car bob up and down slightly when rolling at low speeds with the brakes on.

You can try scrubbing the rear disc down with some wire wool and brake cleaner and seeing if it goes away.

This! If the cars been sitting for a while this normally happens. My rear discs are like this and it gives you sea sickness when braking slowly. You may be lucky and find its just the rears that are buggered when the handbrake has been left on for a long period.