Drum Brake Puzzler

[George7941]

Duo-servo drum brakes have been the gold standard for drum brakes for ever and I expected them on my truck when I bought it. To my disappointment, my truck does not have them. The first pic shows the setup on my truck. I don't think GM saved much money by not having the duo-servo setup

The first pic shows the entire setup. Front of the truck is to the right, you can see the muffler and the tailpipe curving over the rear axle in the background. Second pic shows the top of the front shoe, third pic shows the bottom of the front shoe. The lining is thicker at the top of the shoe than it is at the bottom. Not by much but a bit thicker.

This surprised me since I expected more wear at the leading end of a shoe with its trailing end against a fixed anchor. Any explanation?

These are the original shoes on this 17 yr old truck with all the miles in the stop and go traffic of a congested city.

 

[atikovi]

Not sure what duo-servo is, maybe the power brake booster? Anyway, get yourself the Lisle tool for that W spring to save much aggravation.

 

[meep]

Maybe it’s tightly designed enough that since the top portion moves more, the bottom portion wears more first, and the top follows through and they zero out together?

 

[eljefino]

Not sure what duo-servo is, maybe the power brake booster? Anyway, get yourself the Lisle tool for that W spring to save much aggravation.

You can design brake shoes to jam themselves into the drum when rolling forwards in a sort of cam action. Popular before power brakes.

 

[George7941]

Not sure what duo-servo is, maybe the power brake booster? Anyway, get yourself the Lisle tool for that W spring to save much aggravation.

https://www.hemmings.com/stories/article/duo-servo-brakes

 

[MParr]

Good Grief!

 

[CleanSump]

They worked fine for 17 years, so?

 

[George7941]

They worked fine for 17 years, so?

Just curious. In theory it should wear more on the leading end.

Love Enfields, Matchless and AJS.
Especially the military versions. Would love it if you could post a pic.

 

[CleanSump]

Just curious. It should wear more on the leading end.

Love Enfields, Matchless and AJS.
Especially the military versions. Would love it if you could post a pic.

That's what I figured; you're just surprised and posting out of interest.
I wish I had an AJS or Matchless. Good thing about the Enfields is they kept making the old model in India so parts stayed available.
Not an intentional sidetrack, but.....I put a Mikuni on in place of the leaky Amal.

 

[Trav]

Check these out off an old FJ40, double wheel cyl.

 

[CleanSump]

is that copper tubing or just my 'puter not rendering well?

 

[George7941]

The connecting line between the two cylinders is inside the drum, have not seen that before.

 

[George7941]

That's what I figured; you're just surprised and posting out of interest.
I wish I had an AJS or Matchless. Good thing about the Enfields is they kept making the old model in India so parts stayed available.
Not an intentional sidetrack, but.....I put a Mikuni on in place of the leaky Amal.

View attachment 149413

You even have it in olive green, yummy!
You must have the decompression valve, to make kick starting easier.

 

[CR94]

Friction tends to drag the forward shoe downward, increasing the pressure on the bottom of it

 

[CleanSump]

You even have it in olive green, yummy!
You must have the decompression valve, to make kick starting easier.

Yep Sir!
Kick through twice to prime. Bring to top of compression stroke then release the pressure. ignition switch on, Kick her through.
Starts first kick every time.
You can see the oil filter just above the brake pedal. It just has raw cotton in a perforated cardboard tube in there. Advanced design when it was designed in 1938?

 

[Trav]

is that copper tubing or just my 'puter not rendering well?

It sure looks like it. I would of thought steel was OE. It was probably replaced by the owner, copper nickel would have been a better choice IMO.

 

[zrxkawboy]

Check these out off an old FJ40, double wheel cyl.

Chrysler did something similar on their front brakes. My '57 Plymouth had dual front cylinders. This is from the FSM:

 

[DuckRyder]

Self energizing brakes don't work that great with ABS.

 

[clinebarger]

Friction tends to drag the forward shoe downward, increasing the pressure on the bottom of it

But wouldn't the trailing shoe wear opposite of that, Dragging it upward?

From my experience....Rarely does a brake shoe & drum match up perfectly, The Drum is either oversized or undersized for the arc of the shoe, Which is why back when front drum brakes were standard on most vehicles the shoes were ground/re-arced to fit the drum.

 

[George7941]

Self energizing brakes don't work that great with ABS.

Thanks, did not think about ABS. I now see why GM did not go with the time tested, ubiquitous for a long time, duo servo brakes.