I just checked FAQs, but didn't see an answer.

1988 Volvo 240DL N/A with about 300k miles. It does not have ABS and the entire brake system appears to be stock. My brakes were fine until this morning.

In October 2018, I replaced the brake master cylinder and installed speed bleeders withou any problems until today. Two weeks ago, I bled the rear brakes to make the pedal higher, which happened (surprisingly they were the highest ever on this car). I then refilled the brake fluid to full. No problems with brakes at all until this morning. I have not checked the brake fluid since refilling the reservoir two weeks ago, but never had a brake fluid leak.

I was driving and noticed the brake pedal seemed a bit lower than normal. However, it was not spongy and I had good braking ability.

A bit later this morning, the "brake failure light went on." The brake fluid reservoir was only half full. I added brake fluid. The "brake failure" light went out and the brakes felt fine, i.e. I drove home without any problems (although I can't recall if the pedal went down).

I got home and checked all brake lines, the booster/master cylinder, and bleeder valves (I put a clear hose) with the ignition key in position two. Everything in those areas is as dry as a bone.

However, during the testing process, the reservoir went down a little and the brake pedal sank lower than earlier in my testing the brakes. There's still not sponginess in the brake pedal. On starting the engine, the brake failure light went out.