cougar brakes

Hello all, I was wondering if anyone knew the answer to this one. My brother in law brought me his 98 cougar and told me that his clutch was
acting funny. I looked at it, determined that it was out of brake fluid, so I filled the master cylinder up, and it ran fine for a while. Now it's leaking like crazy; I have to put half of a small bottle of brake fluid in it every week, and I've noticed a leak going all down the driver's side rear brake line. It starts right by the driver's front wheel well. I'd rather fix this problem myself, since I'm sure a mechanic would have a very easy job of trying to rip me off. Any help would be appreciated.
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