It's either ..
the slave leaking .. if internal it will drip out the bottom of the bellhousing.
or the master is leaking .. mostly goes back into the cab and runs under the carpet
or you could have a damaged line.
I would suspect the slave .. on a 91 it's way overdue ..
If it is, and it's an internal type, buy a complete clutch kit and replace the lot . .and don't forget to have the flywheel machined while it's apart.