I own s13 a few years (~10
) and from beginning has stiff pedal and long stopping distance, it's almost impossible to lock wheels on dry grippy road.
I tried rebuild all calipers, new dot4, uncountable times bleed air (with enginge on/off), new brake discs+pads, brake hose and nothing change...
Brake booster is airtight, check valve works.
My brakes are almost stock (93' s13 200sx abs), only s14 front calipers, and from beginning i have anti-lock warining light active (problably faulty abs control unit, because abs wheel sensors works fine on osciloscope).