Toyota Camry: brake squeal

my 99 camry w/4wheel discs squeals when backing up but has never ever done
it while driving forward....is this just brake dust or a weird wear
indicator or what?????? there is 3/16-1/4" of pad left after 90,000
miles....still plenty I assume..

"Richard Bailey" <edu> wrote in message
news:400ff303$utk.edu... 

The very absolute minimum used to 1/8" (GM specs for a compact), anything
less than that cab result in boiling fluid(the pad material acts as an
insulator to stop disc-heat reaching the fluid in the caliper) but I usually
do them at 3/16"-1/4" or so.
Squealing is very annoying and usually is caused by the anti-squeal shim
missing, scores in the disc surface(s), dust and anything that reduces a
perfect 100% contact.

Jason