I too, suspect a hydraulic issue.
The part on the firewall is not the slave, it's the master, which drives the slave. Follow the fluid line down to the tranny, if it runs inside the bellhousing you still have the internal slave configuration. If the line is attach to a hydraulic unit bolted to the outside of the tranny, you have the external arrangement.
In this picture, where it says "Master & Slave Cylinders" the master is the unit like on your firewall (it has the reservoir) and the external style slave is the part to the right in that same picture block.
In the block below that is the box marked "Hydraulic Control Unit" and that is the internal style slave unit, which is a combo slave cylinder/throw-out bearing unit.