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I have a 2001 TJ and when I park facing downhill the parking brake works fine. If I face uphill and yank that thing as high as it'll go I still start slipping backwards (if it's steep enough it's at a pretty alarming rate). Why is this?
 

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The rolling backwards is not uncommon, although I wish I had an answer as to why. You could try adjusting the linkage under the tub, but you'll want to spray it down with pb blaster prior to.
 

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The park brake is a drum brake and only activates one shoe therefore it is inherently directional

Think of the shoes as two half circles split with a vertical line
At the bottom they just pivot and at the top they are spread apart when you apply normal brakes

So the front shoe pivots forward and the back one pivots back.

Now if you are looking at the brake on the left driver side from the outside that front shoe pivot is the way the drum would drag it when rolling forward and the rear shoe pivot is opposite the way the drum drags it rolling forward

That means forward roll with brakes applied the braking helps apply the front shoe and works against the back shoe

That self allpplying is why drum brakes generally don't need power assist

One shoe is called leading the other is called lagging and with normal braking one is always self applying even if backing up

However if you use the ebrake cable it only rotates one shoe not both and the one it applies is the one that self applies with forward roll so the ebrake is directional

Honda motorcycle tried a setup where both shoes were self applying in forward motion but dropped it as you could not control roll back if you failed to make it over a steep embankment as both shoes were only self applying for stoping forward motion

I remember a nurse who said she had to take the bus because she could back out of her driveway but car would not go forward. For the first someone had applied the park brake and her small engine car could overcome the park brake only in reverse because park drum brakes are directional

My park brake on my tj are disc from a Cadillac eldarodo calipers and not directional as the park brake manually squeezes the caliper on the disc
 

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A standard drum park brake is always unreliable to prevent rolling backwards that is what in gear or park awl or wheel chocks are for
 
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