Technical Drum brakes - lack of stopping power

My T-Bucket has been on the road for three years now. My one big complaint with it is the lack of rear brakes. I’ve been pulling my hair out and I’ve done everything I can think of to fix it. I cannot lock up the rear tires, even on the grass or loose gravel. If I am driving at 10-60 MPH and I pull on the parking brake lever as hard as I can, the car just barely, barely slows down. The car stops well on dry pavement, but very poorly on wet pavement or gravel. The rear end and brakes are from a 1979 Chevy Malibu. I built the car with a new master cylinder and wheel cylinders, and I have since replaced them again to make sure that was not the problem. I installed a pressure gauge on the rear brake system, and I can easily get it over 1000 PSI, so I don’t think that is the problem. At first I had the drums machined, and then I put a new one on with no change in braking. I have tried four different brands of brake shoes, including NAPA and AC Delco. They all act the same, even after a 500 mile break in period, and the contact pattern looks pretty decent on them. The only thing I can think of is that the brake lining material is somehow too hard or does not have enough friction?? I do know that in recent years they have removed asbestos and copper from brake shoes. What else can I be missing?! I used NAPA shoes on the front drum brakes, and those work fine.

 

These are self adjusters, no offense but, are they installed correctly? Pics would help here. How are you adjusting the rears initially? Is there a proportion valve in the system?

 

1000 psi is enough to lock up the rear brakes on something like a t bucket. Pictures with the drum off would maybe help. Because the park brake and the service brakes are both crappy it leads me to believe it’s an assembly or adjustment issue.

 

I adjust the self adjusters out until there is a slight amount of drag on the drum/wheel tire assembly. I’m out on the road and can’t take pictures today. I will when I get home.

 

There is no proportioning valve.

 

Try adjusting them out until the shoes are in full contact with the drums and you can't move the wheels. Back them off until they turn easily and then tighten until there is a very slight drag. Just enough that you can hear a whisper of contact. It's time consuming but it works.

 

Now obvious, throwing new parts at it is not solving the problem.
Time to visit the mechanics/operation of the system.
Wheel cylinders need brake fluid to push the wheel cylinder pistons out..why aren't they getting the required brake fluid/pressure to do that?
I'd be looking hard at the brake master cylinder push rod/pedal adjustment, residual pressure valve/s, or proportioning valve, if none exists..research a proper drum/drum system, and duplicate it.

 

I wonder if the rear wheel cylinders are just too small? Assuming the car is the one in your avatar, the weight balance is quite different than the car that the rear brakes were designed for.

As for the parking brake...without knowing what kind of leverage it has, it's hard to say why it's not working right.

 

my experience with drums are limited, but its always seemed that auto adjusters do a worse job than manual adjusters, just in general.

 

That has not been my experience. Automatic adjusters should keep the brakes adjusted well. They adjust when the brakes are applied when backing.

Ben

 

Curious as well. Photos will help a ton.

Is the master located on the firewall or under the floor?

 

A friend had the same problem - turned out that the shoes where installed backwards, so they where not "self energizing".

Frank

 

That's what I was thinking......short shoe to the front, long shoe to the back

 

You may have a master cylinder without residual pressure valves in it. That would let air into the rear wheel cylinders. You could remove the line on the master from the rear brakes and see if the valve is in there.

 

now that is a great idea. I had assumed it was one problem. Maybe in fact it has two separate problems.

 

The shoes being swapped would explain the hand brake not working and the pedal not working where the wheel cylinder size wouldn’t effect the mechanical brake.

 

Normally the brakes become aggressive when the secondary shoe is installed as a primary on duo-servo drums.
Hard to figure lousy rear braking with a mid sized axle, 1000 psi, on a very light car! Some pictures may be helpful, especially plumbing, M/C and wheel cyl. bore sizes.

 

What size master cylinder bore are you using?
I'd start with wheel cylinders that are the same bore as the master.

 

That wouldn't do anything to the emergency brake - that's why my money is on reversed shoes.

 

Could be more than one thing wrong here though.

 

I am using a Wilwood 10 psi residual pressure valve. I have the shoes with the longer brake linings (the secondary shoes) on the rear.

 

Yes I do have those el cheapo style of backing plates and wheel cylinders. I was wondering if that could somehow be the problem.

 

What is the bore of your master cylinder?

Are the rear wheel cylinders held on with bolts, or an annoying round clip?

 

How do the shoes fit the drums? If you put a shoe in the drum it should touch in the middle of the shoe with about 0.002 clearance on each end. They deform slightly on application to give full lining contact.

Also, it has been my experience that most people adjust brakes with self adjusters too loose on the initial adjustment. They should have a bit more than a slight drag on initial adjustment. Most adjust to slightly dragging at one point on a revolution. There should be a slight drag all the way around.

 

If you do have the clip on wheel cylinders, than @moriarty probably nailed it.

Pull one brake assembly apart. Unclip the wheel cylinder, and see if it wiggles or rocks in the hole that it it is in.

If it does, replace the backing plates.

DORMAN 924220, at the auto parts store of your choice, or online. I believe that's the part number for the pair.

 

Then you still have to figure out the parking brake....

 

The master cylinder bore is .937 inches. Yes, I have those annoying clip on wheel cylinders.

 

Have you replaced the rubber line going to the rear ? If the line is collapsed it will still give you pressure but no flow.