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Thanks to everyone for your time in responding. I believe the problem has been correctly diagnosed and repaired. As I drove the car to the location I could work on it I noticed a shudder in the brakes which had not been there before, and which I took to be a warped rotor. This seemed to confirm the hypothesis that a stuck caliper was engaged with the rotor constantly and the friction had caused enough heat to boil the brake fluid causing the brakes to fail. This diagnosis also fit with certain patterns of the problems occurrence; The failure had always occurred suddenly and returned to normal almost immediately and had always occurred after the car had been sitting for about a week and was then driven for about and hour. This last fits with Phil's explanation of the accumulation of water in the front calipers as the water settles and gravitates toward those massive heat sinks. The lug nuts on the driver side front wheel were hot, but to be honest not nearly as hot as I would have expected. I also remembered that when I bled the brakes I bled from only two bleed nipples and had since read there are three. Removing the wheel the problem was more or less apparent. The rotor on the drivers side front wheel was very difficult to turn. It could be turned by hand but with a bit of force indicating the caliper was partially engaged. The caliper was also missing a lower bleed nipple indicating it had not been fully bled in who knows how long. I had been skeptical of the boiling theory because the car was not pulling nor did I smell burning brakes as I would expect. But given all else my current theory is that even the minor contact of the pads with the rotor, sustained at high speeds was enough to boil some un-bled water hiding in the inaccessible chamber of the caliper. I replaced with a brand new caliper and a vented rotor (the normal size caliper was not available and the old rotor was warped). A side note is that the brake lines broke where they meet the fitting going into the caliper. This is because the fitting becomes locked to the line and as it turns it turns the line. I tried to gently work it back and forth with pb, even mild heat to break the bond. In both cases the lines broke. In pulling the brake lines from a parts car I discovered the correct procedure: Disconnect the brake lines where they meet the flex hose and remove the entire caliper. With the caliper and brake lines removed as a unit the fitting on the caliper can be unscrewed allowing the free end of the line to rotate and remain intact. This is a challenge as the two curving brake lines interfere with each other but it is doable with some finesse. Installation is the reverse process. Line were fitted into the new caliper and the whole unit reinstalled. Getting the lines into the right position is tricky but it is possible. After replacing the caliper I was reasonably confident the problem had been diagnosed and fixed but the other possibility, hardened seals in the MC causing intermittent failure, had not been ruled out. I went ahead and replaced the MC with a brand new one and power bled the whole system with high end DOT4 fluid to make sure I got all the evil out, because as JoeKid said, you don't mess around with brakes.
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