acura rl engine revs when stopping!

Keep in mind that if you have an automatic transmission, you're going to destroy the torque converter if you keep driving that car without the wild idle fixed.

Have the brake booster checked.

I'll concurr, you have a vacuum leak in the brake booster's vacuum circuit, or something tee'd off the same line....the wild revving is the ECM commanding a higher idle to deal with the excess air it's not expecting...

I had this once - turned out to be a bad ground wire at the computer. There were other symptoms also - temp guage was erratic.

the dealer mechanic said this could not happen, because of a limiter.

And this "dealer mechanic" couldn't fix it?

YES IT WILL DAMAGE THE TRANS. The sudden shock of the trans engaging has nasty side effects, damaged input shafts, splines sheared off the pump drive, damage to the differential or axles as well. Ever seen a transmission/axle fail during a drag race, that was due to instant shock loading of the part.

With a hard brake pedal and high idle I would say the vacuum line to the brake booster is split or disconnected. Could also be that the vacuum diaphragm in the booster has failed