85 625 csi, engine off firm brake pedal engine running pedal fades 1/2 way down then keeps going to the floor.

can any one tell me what is actually happening with the hydro boost system. and if any one knows what will have to be replaced or serviced?

i already own a master cylinder for this thing that i'm sure isn't the problem.

i am a toyota tech with 26 years exp. but haven't dealt with this type of system before.

thank you