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Having an understanding of what causes that light to come on can be a help. I know it has been so for me. I learned from my Volvo technician.
Some of this may be old ground for you, but since I know not what you already know about the 240 dual-circuit brake system, I must begin at the very beginning.
The Master cylinder has two separate piston/cylinder units in it. They work together and supply pressure to two separate brake cylinder systems.
One circuit operates one pair of pistons on each front wheel, and one of the two rear wheels. That's a total of three pairs. The other circuit operates the remaining piston pairs.
The two pressure lines from the master cylinder go downward to a junction block mounted on the frame piece. You can see the block by looking down the lines. It has a wiring harness and eight pressure lines attached. (Two lines in, six lines out.)
The junction block is made so that the two pressure systems have between them a small piston. It won't move as long as the pressure the two circuits is equal.
If the pressure becomes different, the piston moves, an electric circuit is closed, and the brake failure light comes on.
So when you see the light come on, something has caused the pressures to differ. Sometimes it's a wildly serious problem, sometimes not. A fluid leak in one of the two circuits can light the light, just anything that makes the pressures different.
You may have had a hung piston in one caliper that later came free. Not easy to diagnose. After a light-on experience, and if not problem is found, the switch can sometimes be re-set. Do it, with the engine not running, by stepping on the brake pedal quite hard, maybe several times.
Note that if the junction block leaks, the fluid will likely come out around the wiring harness connection point. The leak is due to failure of an 0-ring that seals the little switch from the pressurized fluid. Check the brake fluid reservoir daily to see if the front and rear internal tanks are different. Look just inside the left front wheel for fluid on the ground.
Hopefully all this is of some help to you.
Good Luck,
Bob
:>)
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