Lexus: GS400 Brake bleeding HELP!!

Hey,
I've just recently changed the front and rear pads on my 2000 GS400, and I
bled the braking system(I've done other Toyota cars in the past with no
trouble); used Toyota pads and brake fluid; started at the right rear
wheel, then Left rear, then the front. PROBLEM is, I got into a hurry and
let the resevior run empty and air got into the line from that point--I
Bled the air out after adding fluid, but when I looked in the Lexus
service manual,it stated that the BRAKE BOOSTER must be bled if this
happens.

Brakes are now sorta wimpy feeling, semi-spongy, will Not pump up, just do
not stop well at LOW speeds--need incresed pedal pressure!! Lexus service
manager drove the car and said he thought they were OK, and that THEY HAVE
NEVER BLED THE BRAKE BOOSTER AS THE MANUAL SUGGESTED. If anyone has any
info or has had a similar problem and maybe could shed some light on the
subject, it would be greatly appreciated.
NOT SURE HOW TO BLEED THE BOOSTER!!
thanks,
Jay

thanks,