so i have had your problem before at auto x.
i lost clutch pressure entirely before and had to make some changes.
i have gone all out and have a OS Giken triple disk clutch with the lightweight flywheel, stainless steel clutch lines, upgraded fluid and the one mod id actually recommend thats not overdoing it as i overclutched my car and now daily driving is .... interesting.
any how,
Nissan 350z 370z G37 G35 heavy duty CSC Concentic clutch slave cylinder
they make one for the oem clutch too so you wont lose drivability, youll go through plenty of cheap oem slave cylinders otherwise and the cost of labor if you plan to continue autox will be more pricey in the long run, if your gonna replace the slave dont go oem!
just my
good luck autoxing!