Brake fluid leak

So last night I lost about 90% of my breaking power. I popped the hood and noticed I was almost completely out of fluid and it was spewing out all over the leaf springs below. I filled it back up to get me to my destination and monitored it very closely. It has since gone down some, so there is a slow leak somewhere. I pulled the driver sides wheel well liner and inspected the lines that run on top of the frame and they all look good. From underneath, I reached up to the bottom of the fluid reservoir and my hand got coated in clear fluid so I think the leak is coming from there. Couple questions. There is a sensor plug on the bottom of the reservoir...what is that even for? And secondly, do I just replace the reservoir or the whole master cylinder? Truck has 303k miles and I've never touched the master cylinder and I don't believe the previous owner did either. Thanks!