unpredictable brakes

I have a 1970 Bug with standard drum brakes.

Recently I replaced the master cylinder, then bled the brakes
using a pressure bleeder (Motive Products), and readjusted them.

Now, the brake pedal works normally *most* of the time, but
occasionally it sinks almost all the way down to the floor when
pressing the pedal very slowly & gently. Any ideas on what
might be causing this unpredictable (& somewhat alarming) behavior?

(Please post any responses to the newsgroup: My spam filter
is a little too enthusiastic these days.)

Chip
San Francisco, CA

SF Briarpatch wrote:
> I have a 1970 Bug with standard drum brakes.
>
> Recently I replaced the master cylinder, then bled the brakes
> using a pressure bleeder (Motive Products), and readjusted them.
>
> Now, the brake pedal works normally *most* of the time, but
> occasionally it sinks almost all the way down to the floor when
> pressing the pedal very slowly & gently. Any ideas on what
> might be causing this unpredictable (& somewhat alarming) behavior?
>
> (Please post any responses to the newsgroup: My spam filter
> is a little too enthusiastic these days.)
>
> Chip
> San Francisco, CA

Sounds likje an internal bleed inside the master cylinder. A piece of
dirt or a rust flake somehow got in there and gets jammed between the
bore and the piston seal?

Jan