Hi,i've got a nissan safari 1990 that has a fading brake pedal when engine is running,so far the master cylinder has been replaced,new rotors all round,new pads,rekitted callipers and new pistons fitted,but still have a fading pedal,the callipers are all sliding freely,theres no leaks,and if i clamp all the brake lines at four corners it's fine,could the propotioning valve at the rear be causing this problem?as it's the only part thats not new,the indicator(!)light flashes breifly as the pedal is initially depressed, any help greatly appreciated.