Quoting DJ.: As I understand it, the sigma clutch slave cylinder lies around the gearbox input shaft (concentric type). The bleeding nipple is at the top of the bell housing so well above that line.
By the way, on my car if the bleed nipple is turned completely open, fluid runs straight through from master cylinder and out into a jar under the car. It makes for an easy fluid change, I just keep topping up the reservoir and wait
Duncan
Although the slave cylinder is at the top of the bell housing as you correctly said, the bleed nipple is below the cylinder, which is why its a struggle to bleed. The bleed nipple should always be above the cylinder.