I've just completed a MC/ABS replacement in a 2001 Gs430 which I will post separately. I learned tons and searched this forum extensively!
Bleed your system first. Do it right, see if your pedal comes back. When was the last time the fluid was changed?
what worked for me was this.
Car off. pump pedal 40x steady pumps, pedal should get hard. turn ign to "ON" you should hear your pump cycle. it will go off after 25-30 secs. Leave "ON" pump about 5-7 times. depress pedal open bleeder to FRONT PASS. do this until no air is observed. Turn key to off. pump pedal 40x. check and fill. turn key "ON" hear pump cycle. wait til it goes off, then pump 5-7x and bleed FRONT DRIVER until no air is observed.
pump 40x. key "OFF" check fluid level. now do back. key "ON" depress pedal open bleeder, the MC pump will do the work. bleed until no air.
key "OFF" pump 40x check and fill fluid. key "ON" bleed the other rear until no air.
last time. key "OFF" pump 40x check and fill fluid to MAX LINE.
take a ride and see if the pedal is less mushy!