Are you out of fluid, or just low? If you're out or very low, like these guys said, definitely locate the leak before doing anything else. Check around the master cylinder, hard lines and rubber lines, as well as calipers. It might not be that hard a fix. When you're sure you have no leaks, fill up and do a flush/bleed. You'll have air in your system which has to be bled out.
Also check your brake pads ... if they're real low, that can make the fluid level drop as well as increase pedal travel. Wouldn't hurt to do a check of all components anyway ... pads, rubber lines, discs, make sure calipers aren't stuck. Brakes are something you want to stay on top of.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)