A lot about brakes

[cj847]

I wanted to post stuff about brakes that I hope can help some folks. Feel free to correct, criticize, clarify add to, ect. I will make several posts as I have time.

DRUM BRAKES.
I find that drum brakes are hated by most people because they are not understood. At the likely chance of showing what I don’t understand, I will try to explain. Also, the information on the internet is sketchy, confusing and disjointed at best. This information is specific to the brakes on 67-72 truck. While most of it can be applied to any drum brake, some cannot.
The first point of confusion is which pad goes where. These shoes are called primary and secondary. The primary shoes has less friction material and goes to the front while the secondary shoe has more friction material and goes to the rear of the vehicle (this is the last time I will use those terms as I find they add confusion). While the brakes will perform whether the long pad is in the front or the rear, the long pad goes to the rear of the vehicle. And here is why:

The pin in the circle is the only thing that stops your truck. If it was not there the shoes would stick to the drum and spin along with them or at least blow out the wheel cylinder. (Like most things automotive, one bolt or pin is all that separates you from almost certain death). When the brakes are applied, the wheel cylinder expands and the shoes touch the drum. The rear shoe is forced against the pin giving it ultimate stopping power. The front shoe is working as well but only with the force applied by the wheel cyl.

The next point is all those springs and holders. The main thing here is to have the right tools and to take pictures before disassembly. Caution on pictures- most drum brakes I have seen have at least one mistake by the last person who worked on it. For example, my truck had the shoes on backward. Be careful as several parts are side specific. You can do a brake job with screwdrivers and pliers, and you will hate it. Even with the right tools it is frequently a problem. Here are the main tools:

https://www.harborfreight.com/drum-b...ece-63640.html

The self-adjuster. This ‘gadgetry” adjusts the brakes when they are applied in reverse. Think about the above example of the rear shoe being crammed into the pin in reverse. Now the front shoe is crammed into the pin. But, more importantly, any free space between the drums and the shoes opens up at the top of the rear shoe. When there is enough space the “gadgetry” acts on the self-adjuster to expand the distance between the shoes at the bottom. This is why when backing out of your driveway, you should come to a stop with your brake instead of just coasting and shifting into drive. This will keep your drum brakes well adjusted.
PS> yes I know that the diamond shaped washer is missing in the picture. (see green circle)

[ATTACH]Attachment 2326758[/ATTACH]

[cj847]

DISC BRAKES

Most people are familiar with disc brakes and they are generally easier to work on than drums. The main differences are how the calipers are mounted. For this story we are going to work with a typical 1970’s GM type system. This system has a caliper mount and the caliper rides on 2 pins. The main point on any caliper system is smooth action. The caliper is inherently self-centering as it is just riding on pins. Make sure that ALL the moving points are lubricated with brake grease. It is quick and easy to pull the caliper and replace pads. But stop, take your time and test every moving part to make sure it has free motion. If you have time, mount the caliper without pads and move it back and forth thru its motion to check for binding. Here are some pics:

[cj847]

MASTER CYL AND BOOSTER

I recently had a frustrating experience with a booster and master that led to me gaining more experience in this area. The main point I want to emphasize is that the center of the brake system is somewhere inside the booster. This point is “fixed”. (ie everything else is adjusted to it in both directions). The key to having proper brakes is to have approx .020” clearance between the booster push rod and the master cyl. This is adjusted on front of the master cyl. Some master cyl have an adjustment nut on front to make this easy. Others have a replaceable pin. I was very frustrated to find mine had replaceable pins, none of which fit with proper clearance. In the below link there is a video that explains it:

https://www.speedwaymotors.com/Singl...Bore,1983.html

The adjustment rod under the dash adjusts the pedal position in relation to the “fixed” point in the booster. It is not meant to take up slack between the master cyl and the booster or change how far you have to press before the brakes start to apply. (This is with power brakes, manual brakes may be different).

[raggedjim]

Nice job!

Thanks, Rg

[don t. - 72gmc]

I recently R&Red my entire brake system. Got a new booster/master as an assembly.
Issue = booster was smaller in diameter than original. The rod was longer than original as well. When I removed the old boost/master, I left the rod attached. marked length w/tape. Glad i did. I ended up using the old rod, adjusted correctly 1st time.

Only thing I don't like about drum brakes is those blasted springs. I aint buff enough to get em off w/o bleeding(smile).

HTH...Don.

[A1971Blazer]

I bought a brake spring tool at NAPA when I was in high school in 1974....
I still have them....
I still use them......
rear drum brakes are more than adequate on a vintage truck with no ABS control

[70STOVEBOLT]

You say the adjustment is on the master cylinder, but I think you meant to say it’s on the front of the booster?

[cj847]

Yes, thanks for that. I have updated the post.

[joeydurango]

Thanks for taking the time to write all this out. I'll reference this next time I need to mess with the rear brakes...

[chiefcfd]

Nice write-up. Thanks, I grew up working on drum brakes. That's why they are still on my "72. I've had less trouble working on drums than disc brake setups.
By the way cj847. I visit Ft Smith for work every once in a while. It was a Cintas Uniform factory near the river. They have been sold recently. The make Postal Uniforms for the U.S Postal service.

[MikeB]

Originally Posted by cj847

I find that drum brakes are hated by most people because they are not understood.

I fully understand how they work, but I still hate working on them. Fronts aren't too bad, but rears are always a PITA.

[Andy4639]

I'm just surprised you used a 70 model for the disc brakes on these trucks when the first year for front disc brakes was 1971. Other than that I like it.

[HWY-STAR]

Hey Andy4639, you just made an honest error reading the disk brake post #2
Cj84 says. 70,s GM system. Not 70 GM system.
No bid deal, just thought you would want to know.

Thank you for the write cj84

[MikeB]

It's hard to tell on the RockAuto site, but it looks like the same 1.25" thick rotors were used from 71-86. But sometime in the 80s, GM started using thinner 1.00" rotors on trucks with the base GVWR. Calipers and spindles were different, too, but pads were the same.

All that said, CJ847's photos and comments apply to most 70s-80s GM car and truck disc brakes.

[1970cstblazer]

If you have a 1970 or older truck, they used these springs to reduce noise and resonance. I used NOS GM springs 3694290 on my 1970 Blazer since the originals were pitted or all together missing.

[MikeB]

I like finned drums. More mass + fins = better heat dissipation.

My 69 C10's axles have been re-drilled for 5-on-5, along with turned-down locators to fit 71-72 drums. I used to hear that re-drilled axles were unsafe, but mine were professionally done on a CNC machine in the mid 1990s, and still look great.

[raggedjim]

Originally Posted by MikeB

I like finned drums. More mass + fins = better heat dissipation.

My 69 C10's axles have been re-drilled for 5-on-5, along with turned-down locators to fit 71-72 drums. I used to hear that re-drilled axles were unsafe, but mine were professionally done on a CNC machine in the mid 1990s, and still look great.

Mike, I had a set done years ago. The first step was to plug the old holes, if you don't do that the holes are so close together you lose some strength. I never had a problem with mine.

Rg

[oneshotkyle]

Great write up… thanks for the effort. This should be pinned

[Accelo]

Originally Posted by 1970cstblazer

If you have a 1970 or older truck, they used these springs to reduce noise and resonance. I used NOS GM springs 3694290 on my 1970 Blazer since the originals were pitted or all together missing.

I was under the impression that the drum springs were only used on the front.
Is that true or false?

[1970cstblazer]

False. My 1970 had them at all 4 corners originally. All 4 were 11" drums.

[1970cstblazer]

Here is a survivor 1969 Blazer

[1970cstblazer]

Another 1969 Blazer:

[MikeB]

Originally Posted by raggedjim

Mike, I had a set done years ago. The first step was to plug the old holes, if you don't do that the holes are so close together you lose some strength. I never had a problem with mine.

Yes, two of the original holes were plugged with round pieces that were tacked in. As I recall, the idea was to heat the flange as little as possible.

If doing it today, I'd call Currie or Strange, but what I have now will outlive me!

[custom10nut]

Great post.
Spot on with the advice and experience comments.