Q: Replaced slave cylinder, can't get clutch to pump up

It sounds like a couple of things could be going on here. First, there may be air in the slave cylinder and/or line which has to be bled out. Sometimes simply opening the bleeder and the slave master cylinder and letting gravity do the work is successful; other times you may need a vacuum pump to draw the air out through the bleeder. Once the air is bled out of the system, recheck for proper operation.

If proper operation is not observed, I would suspect the clutch master cylinder may be faulty and not supplying the hydraulic force to engage the slave cylinder; this would need to be replaced and the air bled out. A professional technician, such as one from YourMechanic, can assist you with inspecting the clutch pedal issue.