Technical Drum brakes - lack of stopping power

Do your front brakes lockup? If they lockup with normal pedal pressure, but rear doesn't I'm perplexed. But if both take more pressure than normal, then I'd be checking the pedal ratio at the master. For power assist around 4-4.5:1 is fine, but for manual I prefer 6:1 ratio. If this is a manual system without a booster, it might have a power ratio instead, and that could be causing your problems.

 

Forgive me for asking, but why exactly is it that you want the rear brakes to lock up? I ask because folks who engineer cars (and those educated in the dynamics of braking) go to great lengths in order to ensure that the rear brakes DO NOT lock up in panic stop. The reason for this is that when your rears lock up your car is not going to continue in a straight line. And if the fronts are not locked up when rears lock, your car will immediately do a 180. If your braking is as inadequate as you say, it would seem that your problem lies at all four corners. In a well functioning brake system, the front brake circuit is doing 75-80% of the stopping.

As someone above suggested, it would seem that your pressure is adequate, but just because you're getting 1000 psi at the gauge, doesn't mean 1000 psi is getting to the wheel cylinder or the shoes. The brake lines are the first place i'd check to ensure adequate flow from the pedal to to the drums. Make certain there are no obstructions in the lines.

I would not use the E-brake as a barometer for the effctiveness of the rear brake circuit. Old cables and levers are an extremely poor substitute for a hydraulic activation system. If you are using a dual circuit master cylinder (you should be) you can always disconnect the front line and plug the port, then test for true effectiveness. But like I said, it sounds suspiciously like you have problems at all four wheels.

Someone suggested a proportioning valve and that is fine but understand that the only function of a proportioning valve is to address the "cut in" point, or the point at which the rear brake circuit is activated. It does not effect line pressure at all.

Someone above mentioned wheel cylinders. If said wheel cylinders are too large, this could very well be where your problem lies. The smaller the cylinder, the more pressure applied at the shoe. A larger bore diameter in the master will also increase line pressure (at both front and rear). I,e: go from 1" bore master to 1.125". In the wheel cylinders, go from 1" to 7/8" or less. These numbers are just for example. Use your existing setup to determine what size is appropriate.

Drum brakes are self energizing, but in many cases they can be a handful to apply enough stopping power at the pedal. I have a 1963 Chrysler 300 (avatar) that still has drum brakes at all four corners. It also has a factory vacuum booster to increase brake line pressure. If you can't dial in the pressure by swapping out the master/wheel cylinders, this may be another option.

 

If the rear wheel cylinders were too small, it would have the opposite effect. Smaller wheel cylinder = more pressure at the shoe. Larger wheel cylinder = less.
This effect reverses with the master cylinder. Larger bore master = more pressure applied to the line. Smaller bore = less.

 

The front brakes work just fine. As I said the car stops well on dry pavement. I don’t need the rears to lock up easily, but I feel like they are not doing much of anything right now. That makes the stopping pretty poor on wet pavement or gravel.

 

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Somethings you shouldn't seek answers for over the internet when someone is willing to do identify the problem for free. But hey, good luck with that.

As far as it goes, maybe we should start a poll. We have everything from the shoes in backwards to wheel cylinders to small. Me, I think it tires.

Now Steve,

How are your tires? Hard as rock, slick as baked dried out rubber gets in the hot Florida sun? After all, it's 4 drums not a drum disc combo. Maybe it's a brakes are good tires fair kind of thing?

 

Also I checked the hydraulic pressure right at the wheel cylinder bleeder valve. It was over 1000 psi.

 

A guy can over think a problem sometimes its the simplest of things. You said you replaced the master did you use a manual and not a power cylinder it makes a huge difference .I have had the best luck with 79 ford Fairmont manual Master cylinders.

 

Actually, the opposite is true. Larger master bores produces less pressure out for a given input force, while smaller wheel cylinders produce less force out for a given input pressure.

 

A problem I have seen in the past is an undrilled
passage where the brake attaches to the wheel cylinder, also the same thing at the bleeder. I have seen brake hoses collapse causing problems, but usually that will cause them to not release. A look at the brake assembly would help, as well as watching them actually move when applied.

 

Yep, when I was working with the bleeder valves I noticed the wheel cylinders rocking back and forth. I did not realize that was a serious problem. I just ordered a new pair of backing plates from Amazon. Thanks for that info. I can’t believe I was not aware of that. I figured there might be some good knowledge available on the HAMB. I can’t wait to hopefully finally fix this problem.

 

I once bought a car that had nearly new brakes all around . but wouldn't stop .guy had all the self adjusters on the wrong sides after a short time the brakes had self backed off . might be something to check

 

Will do.

 

There is so much wrong with this I’m not sure where to start other than please stop and read this to start.
https://www.jalopyjournal.com/forum/threads/tech-week-57-fargos-brakes-101.1229211/

 

I'm glad someone pointed this out already. Also I believe the problem has ben found with the cylinders wiggling around on the plate, so we will see what happens.

 

"The rear end and brakes are from a 1979 Chevy Malibu"

Last time I looked at a rear end of that vintage, the rear wheel brake cylinders were bolted on, 2 bolts, bolting thru the backing plate into the brake cylinder. Why would they be loose. Like WTF. Who's not paying attention. Who's job was it to take a picture?

5 rear brakes, can you spot the FORD in there? Seriously, the wheels fell off the cart here.

Steve, STFU
That's smarten the fuk up for those who don't hear well.
Call Midas and let them tell you what you need done. Or someone else you trust. Everyone's trying but that isn't the problem here.

You are. My bad for saying it. No harm no foul intended. It's a distance thing and the pressure on you.
But ya need to stop, take a quick picture. If you're missing bolts replace bolts. What's this backing plate thing about. I'm not sure what I missed but I was kind of paying attention.

Harsh maybe but who's getting younger here. Where's the pictures? Show us the money shots.

Oh yea, last I looked to replace backing plates, that was a take it all apart, brakes, then pull the axles job as well isn't it? I'm hoping it's a missing bolt thing for you.

 

Pretty sure I asked about pictures 47 posts ago and several others have asked as well.

 

@NoelC Tell us all you don't know about the rear brakes he has.

 

Look Ma, no bolt’s.

 

You're absolutely right, Mark. I shouldn't have jumped on him about the pics. My apologies to the OP.

 

I guess you never looked at the really crappy GM A body rear brakes from that vintage? That's understandable...they were throwaway cars. Part of the cost reduction was replacing two bolts with one snap ring.

here's the backing plate. No bolt holes.

 

In a previous relationship I tended to a Malibu of one year senior of Op’s. That’s how I know.

 

I worked for a construction company for a few years rebuilding equipment, every now and then I'd have to work on work on whatever was busted and needed fixing yesterday. One day I had to figure out why one of the older Ford half ton pickups had very little brakes, as in mosey on down to a stop. After going over the truck front to back I found the cable for the self adjusters had been left off both rear brakes at the last brake job. I adjusted the brakes with the cables hooked up and everything worked as intended.

 

If the rear axle in question is from a "G" body Malibu, yes, the wheel cylinders are held in with a cheesy clip, not bolts so yes, it will "wiggle" around when twisted with a pair of channel-lock pliers.

A few years back I scrapped out a "G" body Elco, one of the rear axle brake cylinders was retained with a common hose clamp instead of the metal clip, funny thing was the brakes worked just fine!

 

For those who have poor reading skills, I stated in the first post that I am out of town now. I can’t take a picture until I get back home.

 

Just curious if there's a bolt on backing plate (that accommodates bolt in wheel cylinders) and the associated bolt on wheel cylinders to replace the crappy clip on arrangement? I too may be experiencing this same issue (mine is a mid 80's s10 rear axle that also uses the goofy clips).

 

Why not drill and tap the ears on the wheel cylinders and backing plates that you already have, for bolts to stop the movement and see if that solves the problem. Rather than throwing more money at it.

 

There is less material there than you think.