Mark, perhaps there's a problem with a piston seal at one of the calipers. If the seal is less than perfect, say, due to corrosion, the caliper can slurp air past the seal during brake pedal release. This was a problem with some early Corvette Sting Rays that used a lip seal rather than a full face seal. The lip can act as a one way valve for contaminants.

If you don't want to take apart each caliper, bleed the brakes and see which one releases air to narrow down the culprit. But if all calipers are original, it may pay to rebuild each one after seven year's service in the harsh northern climate.

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95 E320 Cabriolet, 169K