Vibrating rear brake

[Ypsylon]

A friend of mine has some trouble with her bike starting to vibrate when she uses the rear brake. It's a carbon fibre full-suspension bike and she's tried different wheels with different rotors and two different brakes and the problem has gotten less and more, but it has never gone away completely.

She's made sure all the bolts are tight and the bearings between the main frame are installed correctly, she claims.

Ideas?

[Fibre-Lyte]

Has she tried different brake pads? Calipers nicely lined up with the rotors (i.e. not bent from a crash)?

Does the whole bike vibrate or just a specific area?

[Ypsylon]

She's used at least two different calipers, so I'd guess they had different pads. I think she tried six different caliper/rotor combos all with different amounts of vibrating, but the problem was always there.

She especially complained about the handlebar, so that she could barely hold on, while using the rear brake.

I think the whole thing vibrates, but she doesn't mind the other contact points as much.

[Boonen]

She's made sure all the bolts are tight and the bearings between the main frame are installed correctly, she claims.

Ideas?

I'd check that anyway as it's clearly not the brakes themselves if she tried that many combo's already. Maybe it's a worn bearing?

[Fibre-Lyte]

The handlebar being the most prevelant throws me a bit. Even if the rear were vibrating, on a full susser I'd imagine the pivot points would take some of that away. I'm starting to think it's front fork related. I'd definitely still check all the pivot points and the rear shock still. The only other thing I can think of (barring frame damage) is a loose rear wheel.

[Ypsylon]

She's tried about half a dozen different rear wheels now, switched pads again, and it was better until the pads were broken in, but once they developed their ususal bite, it was back to usual.

I'm not sure if the handlebar is the most prevelant or just the most annoying part.