Absolutely no petroleum based oil should ever get into the hydraulic brake system. If a drop gets in it starts eating up the rubber parts like the rubber gromet between the reservoir and the master cylinder, the master cylinder internals and the caliper rubber parts. Every time I have heard about oil getting into the brake reservoir it always meant rebuilding the entire system from one end to the other.

A friend of mine who works at a MB dealer even says all the lines (even the steel ones) have to be replaced if motor oil got poured into the brake reservoir. This sometimes means totaling the car because of the expense. Some of the brake lines can only be changed with lots of "stuff" disassembled from underthe car.

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1972 450SL
1982 300D Turbo