My son wants the Herald that's been languishing in a garage for the last 8 years. Both brake and clutch master cylinder reservoirs are drained and there is a slick of fluid under the car below the engine. The clutch master cylinder is old and its obvious that its been leaking as it done a great job.of stripping off the paint. However, there's no similar damage to the paint around the brake master cylinder and no obvious signs of a leak a rear brake cylinders or front calipers...........hence the questions in the thread title.
I would say it's leaked somewhere. I have dug old cars out of fields after many years with a full pot of brake fluid. I would guess one of the Steel lines has rusted through in a place that you can't see..
Yes it does evaporate but it has to leak out to present a large surface area first. Get a decent brake pipe flaring kit and replace hard and flexi lines, and probably all the rubber in the cylinders.