It is now several years since most passenger vehicles were provided with a dual circuit braking system. The increased safety which has been obtained by this, has the disadvantage of extra pedal movement and increased pedal force which is then required when one circuit has broken down. Besides the deterioration in braking performance it can give the driver a shock. By replacing the tandem cylinder of conventional type hitherto used with a new type, these disadvantages have practically been eliminated. (http://papers.sae.org/750385/)
A new type of master cylinder called a stepped-bore type. As indicated in Workbook No. 1, the two circuits in Volvo's triangular split braking system are jointly served by the same master cylinder. If one brake circuit fails, about 80% of full braking power is provided by the remaining circuit.
In the transition from operating on both circuits to only one circuit, there is a change that drivers will feel during an emergency situation. This change is the increased length of pedal travel plus the increase in pedal pressure necessary to brake -- due to the drainage of brake fluid from the damaged circuit.
The stepped-bore master cylinder eliminates much of the feel of these transitional changes. It provides the same braking power and about the same brake pedal travel even if one brake circuit is entirely empty of fluid. Experiencing an unexpected increase in brake pedal travel in traffic can make you grow a little older real fast!
A vacuum pump is installed to provide additional air pressure for the power-assisted brakes. Located on the left side of the engine block, this pump provides vacuum to the vacuum brake cylinder (also called power brake booster). A round, flat rubber piece (called a diaphragm) inside the pump provides the pressure. Repairs to this pump are made using a diaphragm repair kit and, if needed, an internal valve kit. (http://www.164club.org/wb2/wb21_pt2.htm)
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