Lexus: Cruise Control Problem

Have 1991 LS 400 having problem with cruise. The light comes on when button is
pressed in but when lever is pulled down to set speed the cruise light goes
out and cruise will not engage. Also the cruise light will not come on again
until car is turned off and restarted . It will only let you engage cruise
once after car is started. I think it my be a vacum leak. Does anyone know
where to look to trouble shoot this problem. THANKS DON

Each time you start car you get ONE chance to engage cruise. Then you have to
turn car off , start car then try again.