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.
After some work at the garage not long after I posted the question it went away for a while, so I thought it was fixed. That was just replacing worn pads and a corroded brake pipe and bleed nipple. They found a little rust on the discs but nothing unexpected in a vehicle that gets driven about once a fortnight, and nothing else, but couldn't replicate the fault. Since then it really hasn't been driven much.
On Saturday I noticed something else that may mean something: If I cut the ignition, the vibration stops instantly. When I first found it I was rolling up to a red light at about walking pace. Knowing I'd have a wait for green, I turned off the key, but unusually this was before I came to a complete stop. I tested another couple of low-speed coasts with the same effect. Does this perhaps imply an electrical or vacuum servo issue?