You've described a classic symptom of a master cylinder that has leaky
internal seals, John.

Mike

John Goubeaux wrote:
> Folks,
>
> Would the Brake Master Cylinder be responsible for the following
> symptoms:  Pedal is hard and there is no pumping needed to stop but if
> pedal remains depressed,  eg such as waiting at a stop light, and
> allowed to move up and down slightly, it will slowly sink to the floor
> over the course of several slight up and down pumps. I have re-done the
> back brakes: drums,  shoes, cylinders,  and they are adjusted and bled.
> I also see no fluid leaks fore or aft.
>
> Someone earlier suggested, as a check on the brake booster, to press the
> pedal and hold and start the engine and confirm that the pedal sinks a
> bit as a confirmation that the booster is working properly. I have done
> this and it behaves accordingly.
>
> It seems pretty evident that the MC is starting to pass pressure, but
> thought I would confirm before I replace another part.
>
> -john
>