Motor Oil in Brake Fluid

This is off a vehicle which is a 1992 Ford Explorer. Customer told me that someone in his household put motor oil in the master cyclinder and he had someone change the master cylinder and then he still feels that the brakes are hard to grab. I would like to think of this job as being complete. In fact, i would have gone as far as replacing the wheel cyclinders in back and also replacing the calipers in front. Not to mention the rubber brake lines in front too. I have a question. Would i have to replace the steel lines to to be on the safe side to? Someone please let me know. Thank you, Jesse

No. Calipers and cylinders I can see if the amount of oil was large and the exposure was lengthy, since the engine oil would tend to damage the rubber. Flex lines, ditto. But a thorough flush-through with denatured alcohol (called "Methyl Hydrate" in Canada) will suffice if exposure was short and minimal, and in any case will be fine for the steel lines.

The steel lines don't need to be replaced, however, they will need to be adequately flushed to remove traces of motor oil, as motor oil will swell the rubber parts in a brake system. I'd recommend replacing the wheel cylinders and the calipers (or at least rebuilding them) and flushing the brake system. Just replacing the master cylinder is far from completing the restoration of the brake system.

The rubber lines should probably be replaced though.