So I just changed the clutch in my 93 240sx and I can’t get the clutch slave cylinder to “reset” and push back so it will sit in its position against the fork. I changed it about 3 weeks ago but when I changed it obviously the new one wasn’t full of fluid when I put it back on. Any help? Thanks.
Have you tried bleeding the system?
i seem to recall that there was something you had to remove before installation. It was for shipping or something. check the instructions (yeah that paper thing) that came with your slave cylinder.
Try to bleed the system in reverse.
Take a syringe, put on it a rubber hose that fits to the bleeder nose on the slave cyl. Fill the syringe with oil, empty the air from it
have somebody push the clutch pedal to the floor
release the bleeder nose on the slave and start pushing the oil trough the system up to the master cyl - when air bubles stop coming out, release the pedal, or pull it up, and push some more oil up, until the air bubles stop coming up on the main cyl
that way you pushed the air up.
it’s not always easy to bleed the system because it’s harder to push the air down 
(take a big enough syringe, and the rubber hose of a vacuum from the engine or fuel hose should be ok… if you find a clear one that you can look trough, that would be even better)