* Transmission case/rear seal failure: This is the most probable cause. A breached seal in the transmission case (particularly around the output shaft) or the rear main seal (where the transmission meets the engine) allows transmission fluid to escape. Since vacuum lines run near the transmission, fluid can leak onto them and be drawn into the system via the vacuum pressure.
* Vacuum line routing issue: While less likely to cause the *presence* of fluid, a poorly routed or damaged vacuum line might *collect* spilled transmission fluid, especially if the leak is relatively slow and steady. This wouldn't explain *how* it got into the system, only how it might accumulate there.
* Transmission cooler line failure: Transmission coolers are common on vehicles like this. A leak in the cooler lines (often found near the radiator) could allow transmission fluid to reach other areas of the engine bay and, potentially, vacuum lines, though it would usually be more obvious.
* Something else completely unrelated: While extremely unlikely, it's theoretically possible that something else is contaminating the vacuum lines and coincidentally looks like transmission fluid. This would be rare and easily ruled out by checking the fluid's properties (smell, color, consistency).
Why this is serious: Transmission fluid in the vacuum lines can seriously damage various engine components and vacuum-operated systems (brakes, power steering, etc.). The fluid can interfere with vacuum operation, potentially causing erratic braking or power steering failure. Further, the fluid can contaminate other engine systems.
What to do: This is not something to ignore. Immediately stop driving the vehicle. Have a mechanic inspect the transmission and rear main seal for leaks. They will need to trace the source of the fluid and perform any necessary repairs. This will likely involve a significant repair bill, potentially including a transmission rebuild or replacement.
