Brake Failure - The Brake Master Cylinder

Today my 25 years old (which almost become an antique) 240GL Volvo Station Wagon, experienced a brake failure (the braking power was severely reduced) while I was driving to Kuala Lumpur. Fortunately, I had realized the failure before entering the highway. So I have to turn back to nearby workshop for an inspection.

There are lots of brake components that can go bad (wheel cylinders, master cylinders, discs, boosters and even brake pads). After a thorough inspection by a mechanic, the brake master cylinder was found to be the problem and the source of the failure.

There was a leakage inside the master cylinder. The brake fluid inside both reservoirs was almost empty. Since it is an old car, there is no brake fluid sensor which could warned me earlier if there was a fluid reduction.

Most car braking systems are broken into two circuits, (Is that why there are two reservoirs?) with two wheels on each circuit to increase safety. In most vehicles it is one front wheel and one rear wheel although some vehicles split front and rear wheels. With a dual system like this, if a brake fluid leak occurs in one circuit, we only loose half of our brakes. We will be able to stop although it will take somewhat longer. We will have to press the pedal further to activate it. In my case I am not sure which circuit was leaked because it was concealed.

Let me show you how it works:

When we press the brake pedal, it pushes on the first piston through a linkage. Pressure builds in the cylinder and lines as the brake pedal is depressed further. The pressure between the first and second piston forces the second piston to compress the fluid in its circuit. There will be a same pressure in both circuits if the brakes are operating properly.

If there is a leakage in one of the circuits, that circuit will not be able to maintain pressure. Here you can see what happens when one of the circuits, for example the circuit rear seal develops a leak.

When the first circuit leaks, the pressure between the first and second cylinders is lost. This causes the first cylinder to contact the second cylinder. Now the master cylinder behaves as if it has only one piston. The second circuit will function normally, but you can see from the figure above that we will have to press the pedal further to activate the second piston. Since only two wheels have pressure, the braking power will be severely reduced.