Occasionally, when I stop at a stop light, my brake pedal sinks slightly. This happens when the vehicle is idling at a complete stop, with my foot on the brake pedal applying light to moderate pressure. It's difficult to gauge how far it moves but it feels like 0.5" to 1" and then it's firm again. I've been unable to replicate this problem deliberately, but it's happened ~5 times in the last month.
I don't see any fluid leaking around the master cylinder / booster area. Is this symptomatic of a failing master cylinder (internal leak)? I found this info using Google that seems to support this theory:
I also get a steering wheel shake on the highway exit ramp (~70 to 20 mph). I understand that this could be due to several things, but maybe it is relevant.