Posted by Ben on March 12, 19100 at 11:37:18:
In Reply to: Brakes posted by Brian M on March 09, 19100 at 07:09:03:
Moot on the newer Suburbans with rear disc, but here's how I look at show/drum brakes.
There are two type on Sub's. Dual action and single action. All 2500's are dual action and have heard both on 1500's.
Shoe brakes work by expanding the shoe into the drum. It then "bites" and tries to rotate with the drum, but one end of the shoe is wedged against a non moving post welded to the backing plate (backing plate is the stationary plate bolted to the axle tube, which is held in place by the leaf springs). Now that the shoe is against a solid post, it acts like a wedge between the post and the rotating drum (which is connected to the wheel/tire/ground). That is a single shoe. Since there are two shoes, the other is for reverse.
Now for dual action, the two shoes are in series, one acting against the other. Primary is the one that touches the drum first and transfers wedging action to the second, which contacts against the solid post mounted to the backing plate. The more you push on the pedal, the more pressure it applies to the primary shoe, which appleis more force to the secondary shoe (which does most of the stopping). There is a compounding effect (mulitplication of force).
All of this is inside of the assembly cavity of the drum & backing plate. The shoes actually rotate about 10-15 degrees within. THAT rotation of the internal shoe assembly is what is used to actuate the self adjusters.
There is another solid post mounted on the backing plate. That has a cable & spring in series attached to a triangular lever arm. One corner is attached to the cable, second to one of the shoes (usually the reverse or in the case of the dual action, the secondary shoe) and the third corner is what contacts the star wheel. When this whole assembly is actuated, it pivots the triangular arm, which strikes the star wheel and rotates it by 1-3 clicks.
The spring in series with the cable is the determinding factor in how much the star wheel will be rotated (can't over thighten). Add in some gunk & rust on the star wheel threads and you can see why the predeterminded limit is not enough to do the job.
The whole internal assembly MUST rotate in order for the shelf adjusters to work.
Can see how they would have changed it for parking brake initiation, but doubt it. It's tried and true for decades.
You have to really stomp on the brakes in reverse to get them to work. Yoo don't have to come to a complete stop. You can accomplish this on a very long reverse run and hit the brakes SEVERAL times if you have the room.
Still best to manually adjust. If the drums don't have the slot knocked out, you can with a cold chisle and hammer (wear proper safety stuff).
As for adjusting with drums off, I don't recommend it. Blind and you'll have problems reinstalling the drums if you have tightened them properly.
If you are going to do this yourself, learn how they feel when tight enough.
If you still have steel wheels, the sheetmetal "safety clips" may stil be on a couple of the lug studs. They just spin off, but the drum won't come off unless they are removed. Put them back on if you wish, but personally think they aren't needed (if they are, your wheel is already left the vehicle and you have more pressing problems... :^)
-Ben