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As some know, I recently acquired a 65 that has had a 4-wheel manual disc brake conversion. I noticed that the rear brake “chatter” when not on the brake pedal but goes away once I get on the brakes. The rear setup is from late 80’s super bird (Thunderbird).

Has anyone experienced something similar and if so what was the fix? Thanks!
 

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What you describe is known as "brake pad knock-back" and is usually caused by rotors that have a bit of lateral runout or have areas of flash rust that typically occurs as the car sits with the pads just touching the rotors. Lots of times the rust is so light that a good application of the brakes wipes it clean and the problem then goes away....until parked again.

Another cause can be calipers that are failing to self-adjust (calipers that rely on disc pads for parking brake function have a ratcheting-type of adjustment initiated by applying the parking brake) which allows the pads to be "loose" on their slides between the caliper and rotor or caliper piston and rotor.

Sometimes, when all else fails, the installation of a 2# residual valve in the circuit, as close as possible to the master cylinder, can resolve the problem without causing too much brake drag.
 
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