I had the brake pads replaced and the rotors turned on my 1999 Lexus Ls 400 about 3 years ago. After a few months, the front brakes began to squeal very loudly. It only squeals when stopping from a slow speed, such as moving in line at a drive-thru window. I took it back to the shop, and they couldn't determine the solution to the problem. In June of 2010, I had the brakes completely redone by a different shop and insisted on pads from a Lexus dealer. About 2 moths ago the same squealing started again. I took it back and the mechanic said that there was a large build up or brake dust on the rotors. They cleaned it off and soaked the pads in some kind of conditioning fluid. The brakes were fine for 2 weeks but now are squealing again. What could be causing this. Thanks for your help.

The cause of this in my opinion is that the rotors were not turned correctly the first time.
and at this point in time it is my suggestion that you replace the rotors and the pads all with NEW -- and this will fix your problem for good.
my next suggestion is to not go to the same mechanics that tried to fix it already. fine a new one.
in fact -- buy all the parts yourself and take them down to the brake mechanics ( or just a good well known shop mechanic ) and have them install the rotors and pads for you --- it will take 30 minutes ( or less ) and you for a fact know now that the problem is fixed and done right.
I have had bad rotor turns from shops and ever since my first bad run with them --- I just pay the extra 10 dollars and buy new rotors ( if the pads ever wear down that far to ruin the rotors -- which on my vehicle they dont -- it is always on my girlfriends car :-) ).
get NEW and you will be trouble free.

c...