|
|
|
#1
|
|||
|
|||
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). |
#2
|
|||
|
|||
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.
__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.” ― Brian P. Cleary, You Oughta Know By Now |
#3
|
|||
|
|||
I did the rear brakes on my 1963 220S 2 years ago. That required arcing and it worked well.
Quote:
Quote:
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. |
#4
|
||||
|
||||
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. |
#5
|
|||
|
|||
Quote:
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. Quote:
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). |
#6
|
||||
|
||||
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. |
#7
|
|||
|
|||
Quote:
Quote:
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. |
#8
|
||||
|
||||
That is interesting. Need to think about that a bit. Thx for the pics; makes it a lot easier to visualize.
|
#9
|
|||
|
|||
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. |
#10
|
||||
|
||||
Quote:
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. Quote:
Quote:
Quote:
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. 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. |
#11
|
||||
|
||||
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.
|
#12
|
||||
|
||||
Quote:
Quote:
Quote:
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. Quote:
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. 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 ) |
#13
|
|||
|
|||
Quote:
Quote:
The car does NOT have power brakes. Quote:
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. |
#14
|
|||
|
|||
Did you bench bleed the master cylinder?
__________________
1972 280SE 4.5 |
#15
|
|||
|
|||
No. The master cylinder was rebuilt and installed by previous owner's mechanic shop.
|
Bookmarks |
Thread Tools | |
Display Modes | |
|
|