howesy replied:

I have seen it before in a car and it may not be the same cause but in his the rubbers in the master cylinder were letting fluid past.
Undo the thing and pull it slightly away from the booster and see if its wet in the back of it. There should be enough flex in the lines so you dont have to undo them.

Member - Phil G (SA) replied:

Do you know what brake fluid has been used during the life of the vehicle?

It is possible that the seals in your brake master cylinder have swollen from the use of DOT4 fluid.

Derek from Affordable Batteries & Radiators replied:

It is a common problem.

Toyota have a spring recall and updated spring on the '99 to '01 series.

Not sure of the exact models or dates but had mine done about a year ago.