Analyze this

[jsbihn]

Ok, I asked this question already, but I am going to to repeat it because of the lack of answers and the current info. (not trying to offend)

I have a 2003 Hummer with 107+k on it. It is stock.
Not long ago I started to see what looks like a whitish to tannish color smoke come from the rear when I exited off the interstate after traveling for 1.5 hours at 85 mph in the southern heat and humidity. There is a kind of chemical smell to it. I thought it was brake dust at first. Ok, well I have noticed that it will do the above mentioned 2-3 times each time I hit the brakes as I stopping hard. If I try to just slow down and do a gradual stop, it doesnt happen (or at least I cant see it). Around town it doesnt smoke, maybe once or twice if I am cruising for little bit and have to hit the brakes hard, but normal driving.... nothing. If i sit in my driveway and run the engine to about 2-3k for a few minutes and then release the pedal. Nothing. I dont even see anything leak. I have even tried revving to 2-3k and depressing the brakes hard in the drive way. Nothing.
I have put some no smoke additive in with no results.
Now i also cant be sure but I dont think this happened before the a/c was recharged (I know recharging is BS, but my mechanic said he had to to find the leak because the leak was so small - that was about 3 months ago and i still have a/c).
Also, the only thing i see dripping from my engine area at any time is ALOT of water. In fact after the long drive i pull into the driveway and look and there is alot of water dripping to the ground and when i look in the wheel well there is alot of water on the frame and what looks to be the engine (though the early morning and the lack of sleep is fogging my memory a little).

I can not seem to duplicate this unless i am going a decent rate of speed and then brake hard.
given it doesnt look like brake dust since at times i have seen it come up from the sides.

also currently my mechanic has told me that my cats are bad. he said this since when i brought it in i told him about the rattle can sound that comes from the under of the car that at times will come and go and disappear something after the truck has been running awhile. without hesitation he stated those are bad cats.
so there is another issue to think about

any ideas