Vibration when brake stepped on at standstill, 2009 GU

[Hodge]

Got a funny one for you folks.
This has happened to me several times, but today was just another episode, so thought I'd ask.
After a drive at high speed (in my case the ring road @ 100km/h) and then after slowing down to stand still (red lights / off-ramp). Every time I step on and off the brake, there is a vibration present. Cannot pin-point where it's coming from it's really unusual and very distinct, definitely not normal. It disappears after a short drive at lower speeds and/or if I'm standing there still for a while.
The 2 common denominators that I can think of every time it's happened are air con was on and day was warmish. I thought maybe the compressor each time it kicks in, but, only happens after a drive at speed as I mentioned above, it doesn't happen any other time.
Other than that the car is functional I don't lose any power or anything like that. Everything works as per usual.
My obvious suspect is air con. But I don't even know where to begin looking, and WHY it happens with the foot on the brake?
Any suggestions will be appreciated!

[92GQ]

Vaccum leak maybe?

[Ben-e-boy]

X2 vacuum leak, I am pretty sure that if you are stopped and at idle and you put your foot on the brake and the rpm drops there is a vacuum leak.
Long shot but try that

[Hodge]

Thanks boys! My mate instantly suggested that as well. But on contrary, if it were a leak, wouldn't it happen all the time, instead of after coming down from a drive at higher speeds ? On normal conditions, I've never seen my revs drop at idle, if brake is stepped on. But there is a starting point!

[Ben-e-boy]

If you had no vacuum the breaks and clutch would be hard as fork. If the leak is slower than the pump can create vacuum it would still be able to keep up but work harder

[Hodge]

Cheers for that mate. Will get it looked at.

[threedogs]

Are they ABS brakes ?????

[Hodge]

Are they ABS brakes ?????

I believe so yes. It's got the bosch thing under the hood.

[threedogs]

That's your vibration or pulsing that's how they work

[Hodge]

That's your vibration or pulsing that's how they work

So the abs system may be knackered ?