My GT has been parked up for a while. i restarted it, took it for a spin. brakes were por and disks were rusty, so I gave it plenty of stop start to clean them. now the pedal is gone really bad , have to pmp like mad to even stop it. no leaks, no loss of fluid. Its goes solid when i pump it with the engine off. Is it the servo or the master cylinder? how to tell which is at fault?