Trev29 has an excellent answer. The dual system powers two front wheels and one rear from each circuit, so you should be safe. Drive so as to minimize hard braking.

Trev mentions the way that the brake pedal will sink under light pressure when the MC is going bad, any of that?

The junction block is a common failure. To find it, look at the two lines leaving the MC, where they go straight down. They terminate in that block.

If there is a leak there, is usually looks like fluid coming out around the place where the wire connects. However, the problem is a failed 0-ring inside, replacing the harness connector can be doen, but won't fix the leak.

Fluid dripping on the ground will appear a little inside of and slightly rear of the left dront wheel.

Keep a sharp watch on the fluid reservoir. You may note that one compartment is OK and theother is low. Is that's the case, and you can't make repairs right away, it is possible to "feed the leak" for a while. Use a 12 ounce can of DOT 4 brake fluid. It's small enough to carry in a vertical position in the car (maybe near the battery), and easy to pour accurately into the reservoir - fluid eats paint. Check and top up every chance you get, and especially before leaving the car overnight.

Good Luck,

Bob

:>)

If it is a master cyl seal, then the pedal will slowly drop on light pressure eg. while stopped at the lights. (Hard pressure pushes out the seals firmly against the MC wall so the pedal won't drop (and the brakes work).)

The light comes on because a pressure imbalance in the system causes the piston in the brake line junction to move over and complete the circuit (grounding) setting off the light.

If the pedal is mushy then you probably have air in the system. The air has to be compressed first (thus the mushy feel) before the fluid can firmly push the caliper pistons. Bleeding of the brakes should solve that.

You should be able to drive home as the Volvo has a dual diagonal braking set-up, so if one side is out the other should still work.

If you can't find any leaks aand the MC seems fine, ie. reservoir levels stay up and the pedal doesn't drop, then the junction piston may have stuck to one side, for whatever reason. Sometimes stomping on the pedal will cause it to re-center.

There's also a possibility that the wire to the junction piston has chafed and is grounding somewhere.
--
1980 245 Canadian B21A with SU carb and M46 trans

probably the master cylinder.