I can't find any external evidence of fluid leak, but must confess to not once checking the fluid level in the hydraulic reservoir in the past 12 years. It was app. 1/2 litre low, and after filling it, the level has remained constant for 3 days and 50 miles or so. It would appear to me that if the fluid were topped up any sensors would respond to that by returning to normal operation. If there is an internal leak, it is a very slow one. The 4matic light still stays on and 4matic won't engage. I'm assuming it is OK to drive the car as long as underhood function valve is in the "test" position? Thanks