Brake failure light

Last Tuesday the brake failure light came on in my 1990 Volvo 240 sedan.
The brakes felt fine, the brake fluid was full. Took it to my mechanic
on Thursday who could find no leaks. He flushed the brake system and
the light went out. Four hours later the brake failure light came back
on. The brakes still feel fine, fluid is full, normal pedal travel, etc.

The master cylinder was replaced last June or July because I was
noticing some brake fade. Brake pads (and I think the front rotors too)
were replaced two or three years ago.

My mechanic opined that either the problem is with the master cylinder
or possibly the brake balance sensor (my paraphrase, I can't remember
exactly what he called it) is bad and there's nothing actually wrong
with the brakes.

Any opinions would be welcome. Thanks!

 

I had this problem in my '89 240 wagon a few years ago... ended up replacing
both the master cylinder and the brake partitioning valve (which contains a
pressure sensor). The master cylinder was about $30, and the valve around
$70 as I recall. Neither was particularly difficult to do, and after
bleeding the brake lines according to the sequence in the manual, all has
been well for years (other than replacing pads as needed)...

 

Thanks!