that does sound like the brake fluid reservoir to me.
Pull the floor plate out and take a look at the slave cylinders, pull the rubber boot back and see if there is any fluid inside them or if any runs out. If they have been leaking you will probably see evidence of that around them.
Also if there is a slave cylinder on the brake if it has mechanical brake assembly check that one also, pull the rubber boot back and see if there is any evidence of fluid.

Any cylinder that appears to be leaking replace it.
when you refill and bleed the system
use DOT 5 fluid only