Wheel Cylinders and Brake Drums
Wheel Cylinders
Wheel cylinders are found inside drum brakes, where they transmit the pressure in the brake fluid via the wheel cylinder pistons to the brake shoes, which move radially outwards to contact the brake drum, and so slow down your vehicle.
Drum brakes, incorporating the wheel cylinders, are mostly found on rear wheels. Wheel cylinders consist of simple pistons inside a barrel, and with time the piston seals and barrel will wear out, or the barrels will become pitted by corrosion. These parts can be replaced by new units or re-sleeved with stainless steel by local experts. If you buy a replacement unit, make sure that the name of the manufacturer is cast into the cylinder, unless you want to put your life in the hands of an underpaid factory worker somewhere in a rice paddy.
It is now against the law to import and sell brake components that do not meet the 2005 South African standards. Court cases against unscrupulous importers are on-going.
There is an impressive array of springs and hold-down clips in the rear drum brake assembly, and only ONE correct way to assemble them!
Brake Drums
The wheel of the car is bolted directly to the brake drum, so once the wheel is removed, the brake drum is exposed. While older vehicles may have brake drums on all four wheels, modern vehicles have them only on the rear wheels, if they do not utilize disc brakes on the rear wheels.
As the brake shoes wear against the inside surface of the brake drum, this surface wears away, and a ridge is formed at the inside edge of the drum. If this ridge is not machined away when the brake shoes are replaced, it becomes virtually impossible to remove the drum in future without damaging the shoes and springs.
Brake drums can be skimmed to renew the friction surface and remove the lip, but ultimately the drum wears too thin and has to be replaced. Most drums are marked with the maximum safe size that they can wear to.
It is very important to make sure that the replacement shoes match the radius of the machined drum; over-the-counter brake shoes will not do if the drums are not standard!
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