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Soft Pedal Under Light Breaking

Brakes

Model: 911sc, Year:1980, Mileage:165000, Type of use:Both Street & Track
I have a 1980 911SC. It has a unique problem that has slowly been developing with the brakes. Under light braking, the pedal will instantly go soft and if I maintain the same presure, it will go all the way to the floor. If I hit the breaks hard, the pedal is hard. If I pump the pedal, the pedal is hard. Is this a symptom of a master cylinder going bad? Is there a proportioning valve in this master cylinder that needs special care in the bleeding process? There are no leaks from either the calipers or the master cylinder. I just rebuilt the front calipers, installed new braided lines, and refilled with blue racing fluid. I used a power bleeder in the typical sequence, RR-LF. This symptom was randomly present before the caliper rebuild but is getting more pronounced.

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