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Drum Brake 620's: Why do automatics have larger rear wheel cylinders?

Cardinal Grammeter

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The automatics have 3/4" while the manuals have 11/16."


The only think I can think of is when you hit the brakes you tend to be in gear (whether manual or automatic) and also assuming you're not "start to spin, both pedals in," the manual produces more engine braking?



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My '73 FSM shows 3/4 auto and 11/16 manual too however my parts call up shows a single part number through '74 a different one '75 through '77 and a different one again, '78 and up (5/8") for the disc brake trucks.


You guess is as good as any. Automatics have torque converters with a 2:1 multiplication built in. This is why the tach drops as soon as you let off on the gas... it's somewhat like it up shifts into a higher gear.

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