Bleeding Triangular (Dual-Circuit) Brakes
Safety and Redundancy with Multiple Circuits
Volvo has always been a safety innovator, and even before technologies like antilock brakes (ABS) were available on road cars, they were looking for ways to make braking safer and more reliable.
On 1800s, 140s, and pre-ABS 240s, one of the ways they did this was with dual overlapping brake circuits. Also called "triangular" brakes because of the brake system layout when viewed from above, this kind of brake system works with two separate sets of brake lines.
Each front brake caliper has two pistons, so each of the separate brake circuits acts on one of the two pistons in each front caliper, as well as on a single rear caliper. In this way, each active brake piston forms a "point" of a circuit's triangle, and the triangles overlap in the front because each front caliper has two active pistons.
When you step on the brake pedal, the brake master cylinder pressurizes both of these circuits at the same time, with each circuit acting on half of the front brakes plus one of the two rear brakes. If there is a problem with one of the brake circuits, the other circuit gives you redundancy, giving you partial braking even if one circuit is torn all the way open.
Bleeding Triangular Brakes
Triangular brakes are overlapping, but the system comes together at the brake pedal and brake master cylinder and junction block. When bleeding triangular brakes, special care must be taken to ensure both circuits are being bled at the same time, or else being bled in such a way that you aren't creating a significant pressure difference between the two circuits where they come together. This means we do not recommend standard brake pedal bleeding or positive-pressure power bleeding at high pressure.
Here at IPD, we've found three methods which work best with triangular brakes: gravity bleeding, vacuum bleeding, or simultaneous circuit bleeding.
Gravity bleeding
Gravity bleeding is our preffered method, but requires patience. Starting with the rear wheel farthest from the brake master cylinder, crack the bleeder screw loose until fluid starts to dribble out. Top off the brake fluid reservoir, then go make yourself busy for about an hour. Come back, tighten the bleeder, top off the brake fluid reservoir again, and move on to the other rear brake caliper and then each front bleeder in turn.
Vacuum bleeding
If you want a faster method and don't mind buying some tools, vacuum bleeding from the caliper also works well. Top off your brake fluid reservoir, hook up the vacuum bleeder, bleeder cup, and lines to the farthest caliper's bleed screw, give the vacuum pump a few pumps to make less than 20 inHg of vacuum, and crack the bleeder screw. When air bubbles stop running out, tighten the bleeder screw, top off the brake fluid level at the reservoir, and repeat for each remaining caliper/bleeder.
Pedal bleeding
If you have at least two other people helping you, you can use a variation on the standard pedal bleeding method which keeps even pressure on both sides of the brake system during the bleeding process.
While one person presses down slowly on the brake pedal, have both remaining helpers crack both rear brake bleeders at the same time, then re-tighten the bleeders before the person pressing the brake pedal down reaches the end of pedal travel. Repeat this process until both brake bleeders have no bubbles. Repeat at each front caliper, cracking both bleeders on one caliper and then re-tightening them simultaneously, before moving on to the other front caliper.
Pressure bleeding, low pressure
If you already have a pressure bleeder, you can pressure bleed if you're careful and use low pressure. Using your pressure bleeder, set the pressure to 12psi or less.