The dual master cylinder splits the brake hydraulic system into two sections, generally serving the front and rear brakes independently.
When the pistons are pushed forward as the brakes are applied, the seals operate against a smooth bore to generate hydraulic pressure. If one side fails, due, for example to a broken brake line, the undamaged side still maintains pressure independently of the failure on the other side. Most of the time the front disk brakes use a bigger reservoir because disk brakes require more brake fluid.
Master cylinders can develop internal leaks caused by seal wear or corrosion of the bore. They may then require double pumping to develop pressure in the effected chamber.
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