Technical Drum brakes - lack of stopping power

I figured out why my buddy who owned a bucket always wanted me to ride with him.... it was so I could look for traffic coming from the right when we slid thru the intersection with the rear only brakes at full lock. He looked left, I looked right.

 

Who doesn't like the pain from a good public beat down. I know I do.

Well, I stand corrected, eat my tasty crow, and offer my apologies to those I wronged and offended.

Clips.
I was wrong. Not the first time but still, that disappointing.
Stupid GM. STUPID me.

Now I'm going to take a hot shower and see how much of this shame I can wash off.

I promise to use a stiff brush. And I'll push hard when I do.

 

Vacuum boosters don't increase brake line pressure.

They reduce pedal effort, this amplifying the potential force that can be applied by the operator.

 

And also notice the shape of the hole the wheel cylinder mounts into that keeps the cylinder from rotating. Even if you did drill holes for the mounting bolts, I think you'll find that the bolt-in wheel cylinders are round where they pass thru the backing plate.


https://www.jalopyjournal.com/forum/attachments/backing-plate-jpg.5623424/

GM had a service campaign kit for this problem years ago. It consisted of a clamp-on retainer for the back of the backing plate to retain the cylinder. By their appearance they at least looked more suitable for the job than the push-on clips. Plus this "field fix" allowed the dealers to affect the repair without having to replace the backing plates and wheel cylinders.

I doubt that GM offers the clamp kits any longer. And if I remember correctly, if you ordered new GM backing plates they were set up for the bolt-in style and you had to replace the wheel cylinders as well. The nickel a car they saved probably cost them $20 to repair.

 

Because the existing wheel cylinders were designed using FEA, to have no more material than is structurally necessary.

Drilling and tapping holes could lead to catastrophic failure, resulting in serious injury, or death.

 

Yea but other than that?

 

I couldn’t figure out where to start with any of that post….

 

But how many of those cars they sold with the crappy backing plates and wheel cylinders on them did GM actually have to replace under warranty? Unless it was a DOT mandated recall, any replaced after the warranty were customer purchased, GM didn't have to pay for being cheap on the front end. Then most of the customers' mechanics probably replaced the backing plates and wheel cylinders with the same crap instead of the bolt in stuff. The simple fact that aftermarket replacement parts are still available means most of the defective designed parts were simply replaced (or ignored) rather then correcting the problem. If you have to replace the backing plates on your hot rod, fix the problem, put new backing plates with bolt on wheel cylinders on your ride.

If I had one of those rear ends in my ride, I know replacing the backing plates (with bolt on cylinders) would be something that would be on the schedule before this summer's driving season

 

I got you, fam.

 

I don't have any personal experience with this type of wheel cylinder mount, but it seems to me that the cylinder would have to rotate a heck of a lot to reduce the output force to an appreciable degree. Drum brake components are not high-precision items, so a little off-kilter contact with the shoes doesn't seem like it would hamper movement or the transfer of force.
Does anyone have any photos of the amount of wear that it takes to lessen braking ability?

 

If you go to the page and click on the part number say (Raybestos WC 37625 a list of what they fit pops up. You scroll that list and find out that they are on one of the more popular "later" rears that guys use right now the S-10 rears. I got out of doing brakes on a regular basis about the time they started using those and have never owned a rig on that list. Mostly G Body, S-10 based and 82-88 Camaro.
Funny thing is that with all the S-10 frame swaps in Chevy AD trucks and rear end swaps in the ones that don't have a frame swap I haven't seen gripes about rear brakes on them.

OBTW I looked this up before I saw our post and saved it because the wheel cylinder bore is .750.

Snooping S 10 rear wheel cylinders 85 Manual brake S010's are the odd duck out and have bolt on 7/8 bore wheel cylinders.



At any rate that issue has to be addressed before he can decide if that is the only issue.

My experience on a T bucket says that unless all the park brake pieces came off the same donor rig you may have to put some effort into getting them to work right. I had a Mustang brake handle and 56 F100 rear and rear brakes under mine (the rear was free and had the right bolt pattern) and ended up having to redo some things to get it to where it held to pass a Texas state safety inspection .

 

In doing a little online research I found that these backing plates have a different axle bolt pattern than any other backing plates. There is no easy way to convert them to a bolt on style wheel cylinder.

According to several online reviews, the Dorman 924-220 part is thicker and stronger than the original backing plate. So this should fix the problem for a long time.

Also my wheel cylinders have 3/4” bores. There is in fact a S-10 wheel cylinder with a 7/8” bore that is a direct replacement. I’m going to install a pair of those at the same time.

 

Another bit of information;
My front wheel cylinders have a 1.125” bore. The master cylinder is .937” and the rear wheel cylinders are currently .750”

 

While I'm no expert on this subject and not wanting to cloud the waters; it seems to me those three numbers are out of proportion to each other.

 

That’s kind of what I thought after I put them all together. I’m thinking bigger rear wheel cylinders will be a step in the right direction.

 

.750"? That's pretty darn small and with the MC you have won't generate much pressure. I think those need to be closer to 1.00" to be effective. Is there an alternative wheel cylinder?

 

Yes. There is a .875” available. I’m going to try that.

 

Are you using a fruit jar master cylinder or a dual master cylinder? HRP

 

It’s a duel master cylinder for a 1975 Mustang. AutoZone part number NM1614

 

They work great if installed right. One thing rarely done is proper lubrication on various points of drum brakes. Having all the moving points clean and rust-free, a dab of lube at each point will ensure proper operation particularly for the self-adjusters. You do need a specialized lube, either a dry film type or silicon-based grease to withstand the heat. DON'T use a petroleum-based grease, it won't take the heat and will melt and contaminate the shoes, plus hold dust that will accelerate wear.

One other thing that many don't address is worn backing plates. The brake shoes rub on the backing plates at three pad points on each shoe, the shoes can wear grooves in the backing plate preventing free movement of the shoes. Running a quick weld bead across the grooves then grinding/sanding the pads back to a smooth flat surface is the fix.

 

I took a brake from this topic...

 

I just took an emergency brake from this thread, and will shoe it out of here in a fluid motion, until I get enough friction that makes me stop and bleed a little, to air out my feelings.

 

I think I slipped a disc reading that. Drummed up old memories.

 

Humchuck perhaps?

 

No pictures, but @Moriarity and I have both seen it, and fixed it, and on more than one occasion.

 

Guys, he's using a master cylinder from a 1975 Mustang, those cars had disks on the front and drums on the rear. Ten to one on the OP's car the brake lines were reversed so the disk line is going to the back wheels and the drum line is going to the front wheels, that's why the fronts lock up but the rears won't.

 

the front and the back both get the same pressure from a dual master cylinder (unless it's step bore).

 

This plot is THICKENING!