Have you tried using a small mirror and flash light to look under the master cylinder? If I remember correctly there is a charging pump for the ASR system mounted under the master cylinder. There is a short section of hose connecting the brake reservoir to the pump. It is possible that the hose could be leaking.
The metal brake lines going to the rear of the car go down along the inside of the frame rail and are hidden/protected by a heat shield I believe. As you say there is not much room to work.

Good luck,Robert

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Mercedes Benz Master Tech - Retired