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Old 05-21-2011, 06:03 PM
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Jim and REM, good thoughts and thanks for the thread. I can't help but believe some folks don't fully understand drum brakes. They are nearly stupid-simple, but they work real well with excellent dependability.

Originally, brakes were strictly mechanical (you could depend your life on them). Then, hydraulics were used WITH an Emergency Brake override (that was strictly mechanical).

Ok, so hydraulics expand two shoes instead of cables and cams.

So, what stops the shoes from rotating within the drums? "For every action there is an opposite and equal reaction." Brake shoes 'float' with those skinny little piston rods.

It's that HUGE pin at the top. The shoes expand, grabbing the drum, and they rotate until the back shoe 'seats' against the "Brake Shoe Anchor" pin. Think about this; if we only had one direction (Forward), the wheel cylinder would only need one piston because the 'back' shoe is always solid against the anchor, even when the shoes retract. The only time the back shoe comes off the anchor is when your car changes direction. Then, the opposite shoe becomes the 'back' shoe, respectively.

Because we rarely go backwards, and the 'back' shoe is normally on the anchor, that shoe lining needs to be longer than the front lining. That's why Primary & Secondary linings are necessary. For the same reasons, we install our self-adjuster cables on the rear brake shoe. The only time the cable has tension is when the car is reversing during braking. And, the only function the cable serves is to lift the ratchet on the star wheel for when the car goes forward again. So, this 'rocking' motion of the brake shoes is necessary to self-adjust. It needs braking in both directions to work.

The article misses a couple points:
* Some self adjusters (like on my Tempo) simply expand the tops of the shoes farther and farther out. They bring the shoes closer to the drums, but the pistons follow and can eventually come out of their bores causing catastrophic brake failure (evidenced by brake fluid leaking out of the wheel).

Bottom star wheels (and self adjusters) compensate for lining wear by expanding the diameter of the shoes.

* Disk brakes have HUGE pistons. That's why the front-brake reservoir in the M/C is always the largest one. Ford uses the rear bowl and GM uses the front of the M/C for the front hydraulic circuit.

The article offers some good ideas but it fails to mention that a combination proportioning valve shuts off the bleeding circuit, to conserve brake fluid.

I firmly believe every car on the road should have a dual M/C because a hose can burst at any time, forewarned or not, regardless of Drum or Disk Brakes. The author must believe that too because he wrote about installing one on his own car. - Dave
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