Previa Brakes

Help! I am having problems with my '92 Previa brakes. The pedal has long travel and seems to get worse with the engine running, the pedal going down to the floor. If you turn off the engine, pedal pressure is restored. I have replaced all of the brake hoses with stainless BF Goodridge hoses as one was split and bled the system but to no avail. Can anyone offer any help or advice? I fear that the servo booster may be faulty, but that costs £500 to replace. Can anyone offer any advice on testing. Help!!!!!!

but I would suggest that you get a brake boost servo from a junkyard and see if that will solve your problem and you will not be out $$$$.