Charles --

Here's my best attempt at an explanation of an educated guess based on a theory (how's that for waffling, dancing, and doublespeakspeak? I coulda worked for the government. :) ).

Your brake system is a double system for safety -- if you have a failure, you still have half a system to stop the car. Volvo's good on safety stuff.

Between the two halves of the system is a differential pressure switch. If an imbalance -- or pressure difference -- occurs between the two halves the switch triggers the light to warn you.

Guess what happens when you bleed the brakes? Yes!! You get a pressure imbalance! :)

And your switch, fortunately, works -- and told you the truth.

The switch assembly contains a double-ended piston. Each end senses the pressure in one half of the system, and will move left or right when one end experiences more pressure than the other.

The piston has a ridge around the center, sorta like a ring on your finger. The switch plunger sits on this ridge. If a pressure imbalance occurs, the piston moves, the switch plunger drops, the warning light comes on ----- but the piston can't move back to the middle 'cause the switch plunger locks it.

You must unscrew the switch from the body, step on the brake (to center the piston), and reinstall the switch. The plunger will again ride on the ridge. Prevailing wisdom says to look into the hole for excess fluid. If seen, a new switch assembly is called for.

Where is that switch?

Follow the two brake lines down from the master cylinder to a small metal block on the frame, under the MC. Notice that it has several more steel lines attached, and a small round cylindrical thing screwed into the top -- with a wire attached. That's the actual switch.

Before unscrewing the switch, clean off the assembly and hit it with an air gun. It's best to clean off any grime that might fall into your hydraulic system and cause a serious problem later.

Unscrew the switch, step on the brakes, look for leakage, screw the switch back in, and connect the wire.
--