[quest]

oh and I just had a closer look at your picture. That appears to be and old leather seal that is almost completely gone, worn out, you can see all the wetness on the outside of the hub spreading onto the inside of the brake drum. this tells me that you should do all brakes and seals and possibly wheel bearings. worn out and out of adjustment wheel bearings will wreck your seals as well. Because it's a safety issue I would definitely check them all, to get proper safe braking you have to do your wheels in pairs, front together and back together but all four is the best thing to do. With that much seal leakage your brakes linings are going to be damaged soon or already are, I would also guess that your wheel cylinders will be sweating or leaking by now as well.