Strange one as when engine not running pedal is rock hard yet when running the pedal falls as though master cylinder seals passing . Anyone had similar ?

How quickly? The pedal will be hard with the engine off, generally a lot softer when running. Shouldn't go to the floor, but may creep down when you press it. Never been sure if this is right tho, but have experienced it several times on different t cars. There is nothing that can't be fixed with a hammer😜😜
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I wouldn't say quickly , brakes work ok just never experienced this .

If you mean that you start the engine with your foot on the pedal and the pedal sinks a bit then that is quite normal. It shows that the servo is working.

On the other hand that might not be what you mean.  John
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