Well... I'll have to admit, my knowledge of how the brakes work, beyond "I out my foot on the pedal and the truck slows down" is sketchy, so if someone has more mechanical knowledge, I'd bow to it...
but, if the pressure is vented at a certain limit, this limit is presumably is affected by how often the brakes are applied (thereby losing system pressure which is then built up again by the engine). If a bus is overtaking a cyclist it has probably had to slow briefly beforehand. If the time between slowing and then passing the cyclist is fairly consistent, then it might be that the time taken for the system pressure to build up to venting level would also be consistent and co-incidentally might happen just as the bus was passing. In effect, saying it was deliberate would be like saying that a driver deliberately flashed his brake lights at you as he pulled up at the back of a queue in fornt of you... Add in the fact that a long bus is passing you for a couple of seconds, so the venting can happen a couple of seconds either way and still seem to be alongside...
I suspect, if you stood on an urban street corner watching the traffic, you'd see that vehicle brake lights tend to come on at the same spots (not precisely, and not everytime, but there would be a trend). Venting airbrakes on vehicles making regular trips might well act the same.
And of course, once you notice something, you go on to notice it more and more as it reinforces your impression.
As I said, I'm happy to be proved wrong on this one by someone with better greasemonkey skills or more airbrake experience than me... RT? You out there?
