A WARNING light on my 1996 Audi A6 tells me that the brake pads are worn or the brake fluid is low. The pads have done only 1,000 miles and the fluid level has not dropped. My garage did a diagnostic check and says the master cylinder needs replacing — but the brakes work fine. Should I replace it or not? DH, Sidcup, Kent
When you put your foot on the brake pedal, a rod connected to the pedal pushes a piston along the master cylinder, which is filled with brake fluid. The piston has a rubber seal to stop fluid flowing past it and so creates pressure in the fluid. Metal pipes connect the fluid in the master cylinder to the brake mechanism in each wheel hub. As you pressurise the brake fluid, you operate the mechanism and the brakes are applied.
All car braking systems are split into two separate hydraulic circuits as a safety measure. Each circuit operates a front brake and the rear brake on the opposite side. This is known as dual-circuit braking and has been used since the 1970s. (Before that, a leak in any of the seals anywhere in the braking system would result in complete brake failure, because the pedal could not create any pressure to apply the brakes; the fluid would simply flow through the leak.)
With this dual-circuit system, a leak in one circuit leaves the other unaffected, so you always have working brakes on two wheels. Nor need there be any fluid loss: what’s happening is that the piston is moving through the fluid but not creating any pressure. But it does trigger the switch that detects the lack of pressure, and the red warning light on the dashboard is activated.
So if your garage has diagnosed a master cylinder fault, get it replaced or repaired as a matter of urgency. Only half your braking system is working and if the other circuit should suddenly develop a fault, you will have no brakes at all.
TIM’LL FIX IT
Tim Shallcross used to train AA patrols to fix cars. Now he advises the Institute of Advanced Motoring – read more from Tim here.
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