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  #1  
Old 02-18-2018, 08:55 AM
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Drum Brakes - when to arc shoes?

Hi, I have a question and it isn't about Mercedes brakes, but they are quite similar. The car is a 1959 Borgward with drum brakes on all wheels. The brakes have been adjusted twice. The brakes have been bled twice. The adjustment has been done per the factory manual and I believe I did it correctly.

This car is new to me and the seller's mechanic told me that he thought the shoes needed to be arced to match the drums.

Two issues:

1. Long pedal travel- maybe 3/4 of the possible distance before it's really done moving. It doesn't really feel like a rock when it stops, but the pedal does feel firm.

2. Not very much braking. If I try I can get the front left wheel to lock. Others, no.

My thinking is that the pedal travel should stop when the brakes reach the drum, which if I have adjust correctly, should be almost immediately and it isn't. If I had short pedal travel then I would be inclined to get the shoes arched. I guess long pedal travel could indicate drums out of round (?).

So I'm looking for thoughts. Do I have one problem or two? FWIW I know the front hoses to be a couple inches too long as the will rub the tire on a hard turn. (I plan to rectify, only just discovered this).
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  #2  
Old 02-18-2018, 10:04 AM
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Been a long time since you've dealt with drum brakes, ehh?

It sounds as though the drums are worn past maximum. You need to mic them to see exactly what you are working with. Once they've become over-sized no amount of adjustment will deliver the proper friction.

"Arc'ing" the brakes matches the arc of the brake shoe to the diameter of the drum. It increases/evens the area contacted by the shoe lining with the drum surface. "Arc'ing" will increase the stopping power but will have little effect on the travel of the pedal once the brakes are properly adjusted.

It is a tedious process as you need to either machine the drums to the same size or arc the individual sets of shoes to each drum.
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  #3  
Old 02-18-2018, 11:17 AM
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Quote:
Originally Posted by Mike D View Post
Been a long time since you've dealt with drum brakes, ehh?
I did the rear brakes on my 1963 220S 2 years ago. That required arcing and it worked well.

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Originally Posted by Mike D View Post
It sounds as though the drums are worn past maximum. You need to mic them to see exactly what you are working with. Once they've become over-sized no amount of adjustment will deliver the proper friction.
Can you tell me specifically why this sounds like they are worn past maximum? What part of my description indicates that?

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Originally Posted by Mike D View Post
"Arc'ing" the brakes matches the arc of the brake shoe to the diameter of the drum. It increases/evens the area contacted by the shoe lining with the drum surface. "Arc'ing" will increase the stopping power but will have little effect on the travel of the pedal once the brakes are properly adjusted.
So what is the long pedal travel coming from? They definitely need a lot more stopping power.

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Originally Posted by Mike D View Post
It is a tedious process as you need to either machine the drums to the same size or arc the individual sets of shoes to each drum.
I probably need both. Supposedly the shoes are new and from a preliminary inspection on rear only, the wear is minimal and only applies to about 50% of the shoe surface.
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  #4  
Old 02-18-2018, 01:36 PM
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I agree with all said however I wonder if they are adjusted properly. With a wheel in the air you should be able to turn the wheel but not freely; it should have some resistance from the brakes. Hard to describe but it should take just a bit of force with both hands to turn it and it should not spin.

In the old days you adjusted this with a brake spoon and the star wheel inside the brakes which pushes the brake shoes out against the drum. A screwdriver worked too.

And as I remember if you bled each brake the brake pedal, when finished, should go down about maybe an inch or so before you had brakes. Not two inches or more.
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  #5  
Old 02-18-2018, 01:52 PM
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Originally Posted by tyl604 View Post
I agree with all said however I wonder if they are adjusted properly. With a wheel in the air you should be able to turn the wheel but not freely; it should have some resistance from the brakes. Hard to describe but it should take just a bit of force with both hands to turn it and it should not spin.
If anything, I might have them just a tiny bit too tight. The rear brakes are complicated to adjust and it involves adjusting the front and rear bottom part and then the front and rear center part and then the bottom and then the center until you get the bottom as far out as it can be without stopping the wheel and then the center until it just makes contact.

The front brakes are easier to adjust since they have two wheel cylinders on each, and only two adjustment bolts for the center for each wheel, instead of 4 like the rear.
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Originally Posted by tyl604 View Post
In the old days you adjusted this with a brake spoon and the star wheel inside the brakes which pushes the brake shoes out against the drum. A screwdriver worked too.
I think that would actually be easier, but this car doesn't have the star wheel thing.
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Originally Posted by tyl604 View Post
And as I remember if you bled each brake the brake pedal, when finished, should go down about maybe an inch or so before you had brakes. Not two inches or more.
Well, it goes down a lot more than 2 inches (non-power brakes). I'm stumped by my ability to lock up only the front left wheel (even after re-adjusting).
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  #6  
Old 02-18-2018, 02:10 PM
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You are way over my head on those brakes. My experience with those is only with American GM products so I have never seen such a complicated brake drum as you have described. Jiminy, how did the typically simple design MB engineers ever come up with that? That really blows my mind.

That said, I believe my general observations about the wheel spin and the pedal travel may still be correct.

Good luck. Would really like to see a picture of the inside of that complicated rear brake set up. You are not confusing emergency brakes with regular brakes I wonder and adjusting them the same? There were always two different ways to adjust those.
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  #7  
Old 02-18-2018, 02:24 PM
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Quote:
Originally Posted by tyl604 View Post
You are way over my head on those brakes. My experience with those is only with American GM products so I have never seen such a complicated brake drum as you have described. Jiminy, how did the typically simple design MB engineers ever come up with that? That really blows my mind.
Actually we can blame this on the Borgward engineers, but I don't think Mercedes from the same period is all that different...

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Originally Posted by tyl604 View Post
Would really like to see a picture of the inside of that complicated rear brake set up. You are not confusing emergency brakes with regular brakes I wonder and adjusting them the same? There were always two different ways to adjust those.
The front brake is the first picture. The big nut with a spring under it adjust the eccentric stud that mores that brakes in and out. There is one of these on the front and rear of each front brake. You just alternate adjusting front and rear until it doesn't bind.

The rear has the same setup, but at the bottom it also has these bottom adjustments. They have a lock nut and an eccentric bolt that you turn to move the shoes in or out. I fabricated a tool to turn the eccentric bolt.
Attached Thumbnails
Drum Brakes - when to arc shoes?-frontbraketop.jpg   Drum Brakes - when to arc shoes?-borgwardbrakebackingplate.jpg   Drum Brakes - when to arc shoes?-toolc.jpg  
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  #8  
Old 02-18-2018, 05:22 PM
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That is interesting. Need to think about that a bit. Thx for the pics; makes it a lot easier to visualize.
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  #9  
Old 02-18-2018, 05:33 PM
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The braking system adjustment you describe is the same as mid 50's Chrysler products.

Each brake shoe has an anchor pivot on one end and the wheel cylinder on the other. The adjusters center the shoe vertically for proper concentric contact with the drum. The adjuster also moves the anchored portion of the shoe outward for the brake adjustment we are typically accustomed to.

Given the shoes can be adjusted for proper concentric contact, the need to arc a shoe is reduced but might be of some benefit.

A firm ish and long peddle travel is usually due to swelling brake hoses / too small a master cylinder bore diameter / too large a wheel cylinder diameter / brake drum flex / backing plate flex / brake master push rod too short.

Not getting much braking effectivness may be from the shoes not making full contact with the drum. Do the shoes show wear along their entire face? ( not having wear in the last 1/2" top / bottom isn't an issue. )

Do you know what the bore sizes should be?

Does this car still have a single circuit master cylinder? While not related to the braking issue, I'd be putting a dual circuit master cylinder on this car for safety.
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Old 02-18-2018, 08:49 PM
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Quote:
Originally Posted by 97 SL320 View Post
Each brake shoe has an anchor pivot on one end and the wheel cylinder on the other. The adjusters center the shoe vertically for proper concentric contact with the drum. The adjuster also moves the anchored portion of the shoe outward for the brake adjustment we are typically accustomed to.
This describes the rear brakes. The front brakes have two wheel cylinders on each wheel. For the rear brakes the position of the anchor is adjusted using the eccentric bolt with the lock nut. There is a repeated pattern of adjusting the rear brakes such that you adjust the bottom adjustments such that it is just about to cause friction, then you back off, the you adjust the top ones so they are about to cause friction and leave them causing friction. Then you attempt to release the friction using the bottom ones again, then back to the top. When you can no longer release the friction with the bottom adjustment then it is set correctly and you adjust the top adjustments so that the friction is released.

The front brakes have two cylinders per wheel, one on bottom and one on top. In this case there isn't an anchor point that gets adjusted, only the one adjustment for each shoe for that wheel. Adjust until some friction then back off, then adjust the other one until some friction then back off.
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Originally Posted by 97 SL320 View Post
Given the shoes can be adjusted for proper concentric contact, the need to arc a shoe is reduced but might be of some benefit.
It would seem that I cannot adjust for proper concentric contact and that arcing is needed. I strongly believe at least the rear shoes are new. I have not seen the front shoes because I don't have the special tool to remove the center hub cover (it unscrews somehow).
Quote:
Originally Posted by 97 SL320 View Post
A firm ish and long peddle travel is usually due to swelling brake hoses / too small a master cylinder bore diameter / too large a wheel cylinder diameter / brake drum flex / backing plate flex / brake master push rod too short.
Does the brake master push rod need to be adjusted with respect to the pedal location? I should go look at this and see if I can see it. I believe everything to be stock and the hoses are new, although the front hoses are too long. It very much DOES feel as I would imagine for swelling brake hoses. I do not know what backing plate flex or brake drum flex mean.
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Originally Posted by 97 SL320 View Post
Not getting much braking effectiveness may be from the shoes not making full contact with the drum. Do the shoes show wear along their entire face? ( not having wear in the last 1/2" top / bottom isn't an issue. )
I have verified that the rear brake shoes are wearing on approximately 50% of their faces. That was observed prior to adjust the brakes.

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Originally Posted by 97 SL320 View Post
Do you know what the bore sizes should be?
According to the manual, the master brake cylinder has a bore of 25.4 mm and a stroke of 36 mm. The front cylinders have a bore of 28.57 mm and the rear cylinder has a bore of 22.2 mm.
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Originally Posted by 97 SL320 View Post
Does this car still have a single circuit master cylinder? While not related to the braking issue, I'd be putting a dual circuit master cylinder on this car for safety.
Yes, single circuit. I'll be happy if I can get that working for now. Picture of strange hub to be removed shown in attached picture. I have since been told by the Germans that it unscrews in a normal manner. hah.
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Drum Brakes - when to arc shoes?-brakeadjust2b_2018-02-05.jpg  
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  #11  
Old 02-19-2018, 08:02 AM
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All good suggestions, particularly those relating to replacement of the flexible brake lines. Generally, arcing brake shoes is not necessary if the proper parts are being used. Need for arcing is shown by wear patterns on the shoes themselves indicating uneven contact with the drums. In this case I am intrigued by a reference to wear "along 50% of the faces," which causes me to wonder of the correct parts are being used and, if so, have they been assembled in the proper order.
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Old 02-19-2018, 05:30 PM
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Originally Posted by ScooterABC View Post

The front brakes have two cylinders per wheel, one on bottom and one on top. In this case there isn't an anchor point that gets adjusted, only the one adjustment for each shoe for that wheel. Adjust until some friction then back off, then adjust the other one until some friction then back off.
Oh, OK, two double ended wheel cylinders. I was thinking of the Chrysler front setup with 2 single ended wheel cylinders.

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Originally Posted by ScooterABC View Post
It would seem that I cannot adjust for proper concentric contact and that arcing is needed. I strongly believe at least the rear shoes are new. I have not seen the front shoes because I don't have the special tool to remove the center hub cover (it unscrews somehow)..
Yep, you need to get the drums off for a look. If the shoes are soaked in oil you would have poor braking as would if the lining is too hard.

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Originally Posted by ScooterABC View Post
Does the brake master push rod need to be adjusted with respect to the pedal location? I should go look at this and see if I can see it.
You want to make sure when the pedal is pushed it is pushing on the master cylinder piston. If the rod is too short you are just pushing against the return spring. There needs to be say 1/16" to 1/8" rod to piston free play. The measurement I'm specing isn't peddle travel, peddle travel will be longer.

Does this car have manual or power brakes? If manual, pushing the peddle with your fingers to take up the slack and feeling for a bump or wiggling the rod until it stops while pushing would be a test.

If this has a brake booster like a modern car, pulling the master cylinder back and measuring rod to mounting surface or booster then measuring piston to surface or master cylinder is needed.

If this car has a remote vacuum booster this becomes more complicated.

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Originally Posted by ScooterABC View Post
I believe everything to be stock and the hoses are new, although the front hoses are too long. It very much DOES feel as I would imagine for swelling brake hoses. I do not know what backing plate flex or brake drum flex mean.
The baking plate is the circular metal disc where the adjusters / wheel cylinders / brake shoes attach to. If this is bent and brakes are applied, the plate will try to spring back into position giving long pedal travel.

If the brake drums are too thin ( like wayy. . too thin ) , they will flex as the brakes are applied. Remember, the brake shoes push outward in 2 semi circles, this tends to flex the drum. Grab the top of a round plastic garbage can and pull outward with hands 180* from each other.



Quote:
Originally Posted by ScooterABC View Post
I have verified that the rear brake shoes are wearing on approximately 50% of their faces. That was observed prior to adjust the brakes.
Is this wear at top , center or bottom of shoe? If brakes are run out of adjustment then adjusted, they won't stop so well until the shoe wears enough to fully contact the drum. ( Back to the arching you talked about )
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Old 02-19-2018, 05:52 PM
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Originally Posted by 97 SL320 View Post
Oh, OK, two double ended wheel cylinders. I was thinking of the Chrysler front setup with 2 single ended wheel cylinders.
I actually don't know if they are double or single ended wheel cylinders - I haven't had the front drums off yet.
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Originally Posted by 97 SL320 View Post
You want to make sure when the pedal is pushed it is pushing on the master cylinder piston. If the rod is too short you are just pushing against the return spring. There needs to be say 1/16" to 1/8" rod to piston free play.
I was able to inspect this yesterday. There is less than 1/8" free play between the rod and the pedal actuator.

The car does NOT have power brakes.

Quote:
Originally Posted by 97 SL320 View Post
The baking plate is the circular metal disc where the adjusters / wheel cylinders / brake shoes attach to. If this is bent and brakes are applied, the plate will try to spring back into position giving long pedal travel.

If the brake drums are too thin ( like wayy. . too thin ) , they will flex as the brakes are applied. Remember, the brake shoes push outward in 2 semi circles, this tends to flex the drum. Grab the top of a round plastic garbage can and pull outward with hands 180* from each other.
I know what a backing plate is I have just never heard of it flexing. Similarly, I have never heard of a drum flexing.

The wear to the shoes I saw from the rear brakes was mostly an area off center from the center. Maybe starting a 1/8 of way up and going to a little past half way up.
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Old 02-19-2018, 09:54 PM
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Did you bench bleed the master cylinder?
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Old 02-19-2018, 09:59 PM
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Did you bench bleed the master cylinder?
No. The master cylinder was rebuilt and installed by previous owner's mechanic shop.
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