Intermittent juddering sensation felt through the pedal on moderate braking

Sometimes, but not always, when braking fairly gently (e.g. approaching a red traffic light) I feel a juddering or pulsing vibration sensation through the pedal. As I slow, the frequency slows, and I could almost believe it’s once per wheel revolution. Braking performance is no more than minimally affected, possibly unaffected. I don’t feel it on hard braking, which I do very rarely, or with the very lightest touch on the brakes. The sensation is much less than ABS pulsing on other cars I’ve driven. I don’t know if this van even has ABS; I believe it was optional in Transits back then.

If it happened all the time, I’d assume something up with the discs (front) or less likely the drums (rear) – like warping or stubborn rust. In fact it feels rather like when I’ve driven it with rusty discs after it was unused for several months. But it doesn’t happen all the time. In fact if I release the pedal completely then re-apply the brakes, there’s a strong chance the feeling will go away. There’s a little bit of buzzing sound in time with the feeling, most detectable when almost stopped. I don’t think it sounds like when the discs were rusty, but that was more continuous.

I’d like to understand the problem, especially as it’s intermittent, though I’m not set up to do significant work myself and would need to get most repairs done at the garage. This is a 2003 Ford Transit (RWD diesel 2.4l), converted to a campervan, that only gets driven every couple of weeks plus occasional longer journeys. This doesn’t happen every trip, and doesn’t seem to be more/less common after a long run. Road surface conditions don’t appear to make any difference.