Fluid lines, hoses and brake fluid
The fluid lines and hoses
The fluid lines connect the master cylinder to the slave cylinders on the wheels of the car.
These are mostly thin steel pipes, and are therefore prone to corrosion. Have them inspected when your vehicle is serviced. Vehicles that are not often used, like collector's pieces, should use a silicone-based brake fluid which is expensive, but prevents corrosion.
Where the brake lines are attached to moving suspension parts, flexible braided rubber hoses have to be used. These hoses develop cracks with age, and should be inspected carefully. As they get older, they are also prone to pieces breaking off on the inside, blocking them. Furthermore they can develop bulges due to the failure of the braiding. This tendency to bulge will cause the pedal to feel spongy, and the hose will ultimately fail. Replace suspect hoses immediately! (And as a matter of routine on vehicles older than 10 years.)
Braided rubber hoses can be replaced with steel braided Teflon hoses, giving a much firmer brake pedal.
Brake fluid
The brake fluid is pressurized by the master cylinder, and transmits that pressure to the slave cylinders at the wheels.
Brake fluid is a very special fluid, capable of withstanding high temperatures caused by severe braking. Never use brake fluid of lower specification than DOT4 - the cost saving is not worth it. Beware of brake fluid in un-marked containers!
VERY IMPORTANT!
Brake fluid has to be replaced periodically - see your vehicle owner's manual
The reason for this is that brake fluid is hygroscopic, i.e. it absorbs moisture from the atmosphere. This moisture content degrades the brake fluid, and also attacks the walls of cylinders, particularly at the wheels, where the higher temperatures promote internal corrosion. Most service centres do not include this in their service schedule, so seek expert advice in order to prevent the degradation of your brake components - an ABS valve block could set you back a fortune!
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