That is true about the Master Cylinder. The brake pedal pushes a plunger in the middle of the Booster into the Master Cylnder, and if there is a leak in the cylinder the vacuum draws it into the booster and over time it can leak in to the cab. The booster is mounted directly to the firewall and if it has plenty of fluid in it and the seal does not hold it will eventually leak in. I had to replace both once in another vehicle and when I removed it a lot of brake fluid poured out of the Booster and the seal was bad too. Brake fluid is hydraulic fluid per se, but the color will tell the truth. I can bet it is brake fluid, have seen it before leaking into the cab.

 

It would be quite unusual, and certainly not likely, but anything is possible. Like bo said, get on your knees and touch the fluid. If it's oilly, it's brake fluid(or clutch fluid, depending on what pedal it comes from).