Axles, transmissions, and transfer cases get hot during use, causing the air and gear oil inside to expand. To prevent this expanding pressure from blowing out the oil seals, these housings use a breather vent valve. On off-road vehicles and trucks, a standard vent cap sits too low. If you cross a deep stream, cold water will submerge the warm axle, cooling it quickly. This creates a vacuum that sucks river water inside, ruining the gear oil. A breather extension hose routes the vent up into the engine bay or tail light box safely above the water level.
These hoses must remain clear of dirt. If mud blocks the hose end, pressure will build up inside the axle housing as it heats up, forcing gear oil past the axle wheel seals or pinion seal, creating leaks.