First I should say thanks to all those who have helped me with my incessant questioning... unfortunately, it's not done... :o and I'm almost at my wits end.

I changed all my brake hoses and the MC today, flushed, bled, everything was operating perfectly. The old MC was indeed leaking, nothing huge but a trail and some rust running down the inner cylinder "tube" or whatever the part that fits into the booster is called. The interior of the booster looked great except for a little bit of nasty stuff at the bottom, not a puddle but some rust and bit of gooey looking residue from old fluid.

Anyway, I got the new master cylinder installed (ATE) , started it up, and NO MORE WHITE SMOKE. Yay. Checked the brakes, drove it, all is well. Still no white smoke on start-up.

Then I drove it for about 30 minutes, let it sit for an hour, and started it up again. A good bit of white smoke came pouring out the exhaust, definitely more than ever before. So I take a look at the cylinder and booster and look what I find...

Sure seems to me that I got a bad master cylinder. Where the hell is that fluid coming from that is seeping through evenly around the seal? Everything was spotless upon installation and follow up. The booster was cleaned lightly with a rag misted with brake cleaner followed by damp cloth with soap and water, everything was dry and I cannot explain where this fluid can be coming from if it's not leaking through the MC. I checked the reservoir fastidiously using an inspection mirror and there is no fluid originating from the "outside" of the MC at the reservoir grommets or anything like that.

Pay no attention to the blue towel, I was about to wipe the fluid away and see if it returns but then I though, NO, this is ridiculous, it's a bad part. I don't know what else to do but try another MC. I very carefully sanded away any flaky rust and paint around where the new o-ring seals while the booster was taped off to prevent any contamination... I was really very cautious about all my steps and the brake system is working perfect EXCEPT FOR THIS... :mad:

I'm sending it back. Where is ATE stuff made today? No clear markings on the part and there was questionable/sloppy casting on the piece which I noted in another post. Maybe they're going downhill?

Unless you're certain the new master-cylinder is leaking, and the white smoke isn't the beginning of a head-gasket failure -
Usually, when a master-cylinder leaks into the booster, a significant amount of fluid collects inside. It may look like there was very little fluid in there, but when you apply the brakes, the booster diaphragm may expand enough to displace fluid into the booster vacuum line.
With the master-cylinder off, check to see how much fluid is in there. You may be able to rig up a small vacuum-hose to shop-vac hose-nozzle and suck the remaining fluid from the bottom of the booster.
Also, some cars have a fluid-trap in that vacuum line. If you have one, yours may be full of fluid.
Happy Motoring, Mark

Thanks Mark,

I should have been more specific: I'm only getting white smoke at startup for 30 seconds or so, and even when the car is in a relatively warm garage: 60 degrees and up. Also, the old MC did indeed have a trail of corrosion and fluid running down the piston after I pulled it from the booster. I inspected the interior of the booster and there was no "standing" fluid. I also inserted a small vacuum hose. No standing fluid.

It sounds like you know more than myself about burning brake fluid; I have little idea how much it takes to generate a bit of smoke. However I did notice that there is some gold/brown (brake fluid?) looking residue in my vacuum line running to the intake manifold. I'm replacing the line and check valve with the newest MC because I know the check valve is likely original and if it goes bad I worry about fumes attacking the booster.

All else aside, how would you explain the clearly wet ring where the MC contacts the booster? I was carefully monitoring the reservoir and there was no spillage to be seen... must be coming from somewhere. Even if the white smoke isn't brake fluid, and it's valve seals, a new ATE master cylinder should not be leaking (though I've read of other people with bad units right out of the box). I'll update this thread with a photo when I pull the current MC.

Often, new parts for vintage cars sit on the shelf so long, the rubber components go bad.
If the new MC is leaking, definitely replace it.
But it doesn't sound like there is enough fresh brake-fluid leakage to cause the white smoke.
You can verify if the smoking is acually from brake fluid by disconnecting the vacuum line from the booster. Plug the line so you don't cause a massive vacuum-leak. Then start it up and see if the white smoke problem continues.
A friend has a '72 280SE and his car starts-up rich. That, combined with some oil-burning from worn valve seals, causes what looks like white smoke after every startup.

Happy Motoring, Mark