I have seen master cylinders fail but not power boosters.
Unless you have an extended warranty that covers 140k miles, then no Lexus warrantly is going to.
Have you tried new aftermarket parts (NAPA, Autozone, etc). I would assume it would be the same booster for a 98 Camry.
steviej
The Lexus has Traction Control which make a difference for brake booster. Camry does not offer Traction Control.