I have been battling with this problem also!
Apparantly it is more common on the low mile / stored cars! Ironic huh!?
I've got a 98 GS300 with 42,000 KLMS.
Anyway, point is this, I just managed to source a complete set, used for AUD $1,100.00
Fitted it (the whole 5 peice unit less actual master cylinder) and it worked, removed the noise, warnings etc. - after proper bleeding.
Only problem!? Its leaking.... bigtime!!
SO!....... Have removed and got refund, am today putting all the original gear back and will re-bleed brakes. (I am unable to find another used set - except for a car thats hit in that guard so not risking it).
Once I've done this I have another solution, I believe it is not the ABS or the unit / master cylinder that needs replacing, but maybe only the accumulator. This may run out of Nitrogen over time and this is why its not building pressure and is working overtime - therefore causing the warnings & errors, but without loss of brakes.
You can buy this part serperately (the brass part to left of ABS setup) for AUD $700.00 brand new from Japan. (once again can't get in Australia!!).
Will let you know how I go and hope the old gear will go back together! IF NOT I'M TRULY STUFFED!!
Look forward to any advice / feedback etc.
Cheers
JJ