Final status report. The symptom re-occurred. I removed the brake booster and discovered the problem. The pivot at on the bottom of the aluminum booster frame for the arm that transfers the brake pedal pressure to the booster was badly corroded and the arm would not pivot freely. I removed the pivot pin, cleaned the pin and frame and the brakes now work and feel super. Again, thanks guys for you help.

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Jim Villers
190SL, 230SL 5-speed, 95 E320 Wagon, 01 E320 Wagon, MGB, Boxster 'S', 190SL "Barn Find"