I'm slowly putting my engine back in. When I took the old one out, I unbolted the clutch slave from the bellhousing, left the pushrod connected to the clutch and tiewrapped the slave to the chassis.

Now, several months later I'm trying to put it back together. However when I attempt to put the pushrod back into the slave cylinder, it doesn't go in far enough for me to bolt it all up. It is as if the piston has moved out. Is it possible that fluid has drained down into it? Would opening the bleed nipple be a good idea or a disaster?

Regards,
Dave

You should be able to push a clutch slave piston back.
The fluid should freely flow back into the master cylinder.

Have you been playing with the clutch master cylinder or pedal?
If the master cylinder push rod is adjusted too tight then the ports in the master can be covered thus stopping the fluid returning.
It is also possible for the master cylinder seals to fail and block the ports.

Alternatively you may have popped the slave piston out too far and the piston has jammed when you have tried to re-fit the slave to the engine.

Try removing the fluid reservoir cap. If that dosen't work open the nipple.