I'm having a problem with my '87 560SL (70k miles):

After cold start-up, the first application of brake pedal pressure produces a loud "pop & groan", the pedal goes spongy and braking is insufficient. Then the ABS light comes on after driving a few feet.

I checked the booster by applying pressure with the engine off, then starting the engine with pressure held on the pedal. The pedal drops slightly so I assume the booster is ok. Fluid level is ok and no fluid is leaking.

My guesses are caliper or master cylinder, but I haven't yet dug into it. Has anyone experienced the same? Any guesses?

Thanks
Frank