How to link an FX property of multiple layers? I'm evaluating an after effects plugin , which has an FX property brightness and similar. I have several layers on top of each other, each letting a portion of the others shine through.

Now I want to animate these FX properties on each layer. Is it possible to create some kind of "master" keyframe control that then triggers the same FX property on all layers?
I think so.  Add a Slider (in the Effects window) to one of your tracks, then use the pickwhip to parent the value of the slider to each of the other Brightness controls.

Examples here of using Sliders and pick whips:
http://helpx.adobe.com/after-effects/using/expression-basics.html