sometimes my accord 93 wagon brake pedal sinks towards the floor and most of the time it does not.

had brakes checked and nothing wrong found... yesterday had brakes bled and new high grade fluid installed... but today the bugger is still doing it, after about 20 minutes driving.

Saw a post saying the problem went away after vigorous cranking of hand brake... something about settling the rear linings...

Saw posts saying check the master cylinder... But my garage is pretty damn good and would not have missed something like that....

Any other ideas ?