If its gear oil, your vent hose might be clogged. There should be a cap on the end of the hose also, it is a vent cap and that might be bad or clogged also. In any enclosed structure with moving parts and fluid you will get splash and preasure build up. If the vent is plugged you might even get seal leaks on the axle seals. Some seapage is normal if the hose doesn't have a good enough seal or the axle was over filled at one time, but it is good to check the hose anyway. I would tell you a trick we use to use in the shops about tasting the fluid to see what kind it is but I think that messed me up over the years. Gear oil stinks badly. With the description you gave it sounds like gear oil to me. Best bet is blow air through the hose while its pointed away from you and if you have a cap on it soak the cap in water with dawn dish soap in it. After soaking for about 30 to 45 minutes run fresh water through it, Make sure the cap is clean of all soap and debris before putting it back on. Some of the caps have small slots in them, these are the better ones but some are spring loaded and any dirt or road grime in them causes them to clog.
As far as the brake line block it is a hard part, they can leak over time especially with vibration, You can try to tighten the lines and examine the block for any cracks. If the brake line block is leaking enough you will get brake fade on the pedal. Being it is on the rear you will only notice it slightly. Brake fade is when you are at a stop with the brakes fully applied at a normal braking preasure, your pedal will slowly move to the floor, being in the rear it will be very slight and hard to notice.
Hope this helps, I use to hate working with axles not only did my Fiance stay away from me but even my Beagle hated the smell of the gear oil.