the slave cylinder fluid is shared with your brakes so dont let it run low or you will lose all your brakes. Slave cylinder is a gearbox out job so i would definately change the clutch and flywheel whilst your there. I have a stage 3 clutch in my st170 and it drives just as a standard clutch, i never noticed it being any heavier on the foot, well worth the money, cheaper than Ford original equipement too.