Master Cylinder.  The rubber seals in there sometimes exhibit this
sometimes sealing, sometimes not symptom.  Especially likely if you find
the level of brake fluid is not dropping.
Edward

At 08:51 PM 7/29/2006, you wrote:
>I've been driving Mellow Yellow around a bit lately. I just returned
>from a four-night camping trip (without Mrs Squirrel, alas, she had a
>conference in Louisiana to attend) up at Big Bear Lake, in Southern
>California. Thing about climbing up to camp at 6,500 feet, is that one
>must return to a lower elevation when the trip is done. This requires
>going downhill, for a considerable distance.
>
>While trundling around the lake, exploring the area, I was surprised a
>couple times by the brake pedal going to the floor. I could brake if I
>pushed a bit, and could pump the pedal up for normal operation if I
>wanted to. After exhibiting this behavior for about five minutes,
>everything returned to normal. But it was disquieting: when one is
>expecting a long downhill in a couple days, any change in behavior in
>the brake system is cause for concern.
>
>Obviously I did not plunge off the side of the highway into a deep
>ravine and die a horrible death of screaming metal and fire, else I
>would not be writing this, showing my stunning lack of knowledge about
>things automotive. The brake system performed normally the rest of the
>trip and brought me home safely. Mrs Squirrel will be pleased, I reckon.
>Before I take her out for our next trip together, though, I bet I better
>have this mechanical issue put safely behind me, so . . . what to look for?
>
>--
>
>Mike "Rocket J Squirrel" Elliott
>71 Type 2: the Wonderbus
>84 Westfalia: Mellow Yellow ("The Electrical Banana")
>74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
>KG6RCR