The brake cylinder is that component of the drum braking system which presses the brake shoes against the external drum surface. Its damage causes the braking fluid pressure to decrease and the fluid to leak onto the friction lining. Its replacement requires removing the wheel, shoes, hoses and, finally, the cylinder itself. New components are fitted in the reverse order. Following the repair, the system should be vented. A good idea is also to clean the entire braking system, check the lining thickness and the condition of the drum. You should always have both cylinders changed as they have a similar useful life.