I've been noticing a problem lately....

first thing in the morning, when i roll back out of the parking spot and hit the brakes, something strange happens. I can feel a thud noise, and the brake pedal sinks slightly, thats it...after that brakes are normal. the pads are new and there are no warning signs......

what could it be? it feels like something is stuck or moves and i cannot figure it out,,,,

Ideas?

Thanks,

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1993 300E, 2.8 M104 .....