True, in a perfect world, the brake fluid would never need to be topped off. But here's a scenario where it might need to be done (and a likely scenario I might add):
Front pads wear down to minimum -- rears are fine. Front pads are changed, piston's retracted with fluid allowed to bleed out at the caliper -- so now there's less fluid in the sytem. Granted, the reservoir SHOULD be topped off at this time. But if it wasn't, then as the new pads continue to wear, the fluid level drops to below spec., and the warning light comes on. Add fluid, and the problem is solved.
Obviously the fluid should be checked when the brakes are serviced. And I agree that if you get the light, it's a good idea to inspect the entire braking system. But when you're looking over the system, don't ignore the obvious.
Just my 2-cents -- arguably worth that.
'92 Mercedes 190E/2.3 (247K miles/my daily driver)
'93 Volvo 940 Turbo Wagon (263K miles/a family truckster with spunk)
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