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Old 09-14-2006, 11:01 PM
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cucumber1949 cucumber1949 is offline
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Regarding the observation of only one side of the wheel cylinder extending when the drums are off:

With the drums off, one side will push out first. This is sometimes misleading because you think that is also what happens with the drums on. With the drums on, the one side will push first, but eventually that shoe will come in contact with the drum, not allowing it to push any further on that side. What happens next is that continued pressure to the wheel cylinder results in a pushing reflex back through the wheel cylinder and begins to push the other shoe toward the drum. When this second shoe contacts the drum, it has a 'servo' action, that is, it gets dragged around a few degrees with the rotating drum and eventually kind of 'wedges' itself into the drum, thereby multiplying the effect somewhat. This is especially true of rear drum brakes.

With the drums off, all the wheel cylinder is doing is pushing out the one side, perhaps until you eject the piston out of the cylinder if the springs are a lot stronger on one side - essentially there is no drum there to limit the travel of the one piston and start the 'extension' of the second shoe.

Older front drum brakes on some cars had two wheel cylinders on the front, thereby allowing both shoes to go into this servo effect, essentially 'biasing' more braking to the front for weight shift, and to prevent the rears from locking up first, possibly causing a control problem.
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