What you're describing could be fluid leaking past the rubber seals
inside the master cylinder.

This is sometimes caused when bleeding the system the "Old" way by
pumping the brake and holding it while an assistant opens the bleeder
... allowing the brake pedal to sink causing the internal seals to rub
in areas where they don't normally go.  Any ridges therein can damage
the rubber.

Mike

Kim Brennan wrote:
> So, in driving the GoldBrick (86 Syncro Passenger Vanagon) recently,
> I've encountered a new issue. When I brake, if I'm lightly braking,
> the pedal slowly sinks to the floor. I don't notice it sinking if I
> suddenly hit the brakes, or it I pump the brakes.
>
> No leaks at the wheels. Fluid level doesn't seem to be falling. So
> I'm thinking master cylinder. Any other diagnostics to look at to
> confirm (or refute) my diagnostic? Clutch seems fine.
>
> Note, the brakes were worked on not too distantly in the past, shop
> put on Big Brake kit, and I asked them to change the fluid then too...
>