TheSamba.com :: View topic - Drum brake pulsation--uneven contact between shoes and drums

Tue Aug 03, 2010 12:16 pm

I've posted about this before, but . . . now I'm serious about getting to the bottom of it. My diesel 82 is down while I gather parts to swap in a 1.9 td, so I intend to address some issues like this brake pulsation problem.

I paid someone to replace the drums/shoes/cylinders/hardware, maybe two years ago. I'm not sure now if the problem was there from the beginning or not. The brakes feel like something is warped, with uneven stopping and pulsation in the pedal. I've verified that it is the rear brakes by stopping with the e-brake--it is very uneven.

My cars have always had four wheel disk brakes, so I'm still kind of confused about these drum brakes.

I removed the drums and had them turned, but discovered that they still made uneven contact with the shoes. The drums would spin freely and then catch at a certain point in the rotation, rub against the shoes and then spin freely again.

Both sides did this--the driver's worse than the other.

I suspected that my drums might not have been properly machined from the beginning; that perhaps the lug holes are not centered. So I bought another set of new drums.

A few days ago, I pulled the rear drums off and tried fitting these new drums. I'm having the exact same problem. The drums catch the shoes at a certain point in the rotation.

So I took everything apart. The shoes appeared to be properly installed. I'm not sure what to look for. I will try a new set of shoes, but I doubt that this will make any difference.

Is it a problem with the hubs? the backing plate?

Thanks for any help. I'm really confused.

Tue Aug 03, 2010 12:44 pm

Backing plate. Most likely a victim of the screwdriver pry off method. Bends the plate and wastes your time.

Tue Aug 03, 2010 12:58 pm

That makes sense. Do you think that I can straighten it, or will I need a new one?

Tue Aug 03, 2010 3:07 pm

Give it a shot, you have nothing to loose.

Tue Aug 03, 2010 4:58 pm

Kaiser Soze wrote: Backing plate. Most likely a victim of the screwdriver pry off method. Bends the plate and wastes your time.

Can you help educate me - I don't clearly understand how a bent backing plate can cause the pulsating brake?

Tue Aug 03, 2010 5:36 pm

mtnwater wrote: Kaiser Soze wrote: Backing plate. Most likely a victim of the screwdriver pry off method. Bends the plate and wastes your time.

Can you help educate me - I don't clearly understand how a bent backing plate can cause the pulsating brake?

Study the post, plate is round,drum "orbits".If bent then an oval, a "high" spot if you will.
Uneven contact.
Bend the plate forward upon contact.
Or just get a new one.
K?

Wed Aug 04, 2010 7:48 am

FWIW, I had a similar problem. What I found was corrosion on the center hub did not allow the drum to center properly. I cleaned up the hub with some emery cloth, fitted the drums, installed the lug nuts to center the drum then install the small bolt on the drum to hold it place.

When you rotate the drums by had it should feel even throughout.

Wed Aug 04, 2010 2:57 pm

kobylan-

I think that my problem is the same! I've been looking at the brakes--it seems like even the new drums 'wobble' on the hub, apparently because they do not fit onto the center part of the hub. I've sanded away the rust from the hub and even sanded the new drums a little, but the fit is not clean.

I'm not sure why!

Wed Aug 04, 2010 6:34 pm

I don't quite get what you mean by fit is not clean. That area is prone to rust and is typically why the drums are hard to remove. I had one side where the drum was really tight and didn't sit flat on the hub. I ignored that and used the small bolt to suck it in. The drum was ever so slightly offset so I had a hugely noticeable pulsation. The key point was that the drum was really tight going on. I don't remember how much I had to clean but after I was done I was an easy fit around the center hub and the drum was flat on the bolting surface.

To center the drum I used the lug bolts then held it everything in place withe the small bolt in the drum. You also need to make sure the hub mating surface is clean and apply never sieze (I used the copper colored stuff) to the entire hub face and center. It will make removing in future much easier.

Good luck.

Wed Aug 04, 2010 8:17 pm

Ok, thanks. By not 'clean' I just meant that the drum wobbles a little. It does not go completely flat against the bolting surface. I'm almost totally certain that this is the source of my problem.

The thing is that I sanded all of the rust away, and then some, but the rotor still does not fit well. Maybe I've been too conservative with my sanding, but it seems like when I'm looking at shiny metal, the drum should fit against the hub.

Another possibility though--I did notice that there is a little bit of a lip on the hub, probably from where the last drum pressed into it. This might be the problem.

I'm just concerned about too much sanding, because if something else is causing the drum not to sit flush against the bolting surface, then if I sand too much it might not get properly centered.

Wed Aug 04, 2010 10:36 pm

Many barke shoes need to be radiused to get them to fit the rear drums properly.
Old school technology which is the solution for what you are describing.

Thu Aug 05, 2010 8:33 pm

Terry Kay wrote: Many barke shoes need to be radiused to get them to fit the rear drums properly.
Old school technology which is the solution for what you are describing.

As right as rain, TK. New shoes should fit new drums, but, anymore, who knows? And, as you said, in the days when brake drums were the state of the art, new shoes were radiused to match drums which had been resurfaced. But, try to find a shop today to radius the shoes.

The last time I had drums cut, I went to the oldest brake shop still doing business in my town. They resurfaced the drums just fine, but when I asked them to radius the shoes, they said no can do. They still had the radiusing machine, it was just that they had heard something about airborne asbestos. They told me the shoes would eventually wear in to match the radius of the drum. Sure, they would wear in - about the time one end of the shoe is at its wear limit and the other end is just starting to get scuffed. Until then, very little of the shoe will make contact.

Being the pain-in-the-ass that I am even to myself, I fabricated a radiusing fixture, moved my 12" bench disc sander outside and held my breath. Worked great - time will tell if I sucked some air at the wrong moment.

New drums might even need some massaging. My buddy, The Tube Guru, noticed some brake pulsation on his new air-cooled Mexican Beetle. The front discs were right on the nuts, so we pulled the rear drums. Both rear drums indicated .012 runout. I trued the friction surfaces, balanced the drums and radiused the shoes. The Tube Guru was happy.

Moral of this story, check the runout even on new drums and don't be surprised if the whole production run is bad.

Richie

Thu Aug 05, 2010 11:54 pm

Rich,
Sometimes you manage to slam me with some old school tech, that worked well,and still works very well--but nobody even comprehends the concept except a chosed few--you being on top of that comprehesion totem pole.

Cutting the shoes to match the drums is the way to fly so your foot isn't doing the Desi Araz the Ba, Ba Loo-- every time you hit the brake pedal---

This is somthing the Vanagon Ute's should look into for pulse free hooks.

Fri Aug 06, 2010 5:57 am

Terry Kay wrote: Rich,
Sometimes you manage to slam me with some old school tech . . .

Gotta thank my old man and the boys at City Brake for my drum brake education, TK.

My dad took to automatic transmissions and power brakes like a pig takes to slop. As a result, I spent much of my youth waiting with him while the guys at City Brake did their magic. They always said they never saw anybody as hard on brakes as my old man; he drove his big-assed 98 like it had an on/off switch, so every SIX months when the safety inspection came due, he'd be at City Brake getting binders all around. I think the owner put one kid though law school and another through dental school on my old man's trade alone.

Richie

Tue Oct 18, 2011 10:22 am

Replaced the shoes and drums on my rear and got major pulsation. Turns out I had the adjusters oriented wrong on the shoe. The big part is supposed to go behind the parking brake lever--I had it in front so it wasn't making contact with the shoe itself, rather the parking brake lever. Took everything off and reinstalled per the Bentley--now no problems.

Tue Oct 18, 2011 1:37 pm

RicoS wrote: Terry Kay wrote: Many barke shoes need to be radiused to get them to fit the rear drums properly.
Old school technology which is the solution for what you are describing.

As right as rain, TK. New shoes should fit new drums, but, anymore, who knows? And, as you said, in the days when brake drums were the state of the art, new shoes were radiused to match drums which had been resurfaced. But, try to find a shop today to radius the shoes.

I can attest to this solution... I had problems after replacing the rear brakes with all new components... ended up finding a local shop in Seattle (Tec Shop) which sold me on the idea that the shoes need to be "ground down" on the tips in order to match the internal radius of the brake-drum... couple days later and I got them back, working just fantastically.

Tue Jan 16, 2018 1:38 pm

After all these years, I'm still having this problem, and there's a twist:

The van was sitting for about a year and I've just recently come back to it. When I left it, the rear brakes were about as good as I could get them. I went to a very knowledge local brake guy and had him turn my drums for me again. They were only very slightly out of round. I put everything back together and the pulsation was present, but minimal. It varies depending on the relative position of the rear wheels: e.g., after a turn, it will be either better or worse. I've assumed that this is because the extent of the problem varies somewhat depending on whether the uneven roundness of the two rear wheels is or is not aligned.

But here is something interesting: after sitting for a year, the problem is much worse. One of my tires had gone flat due to a bad valve. I had this fixed and replaced the wheel.

Now there's a very bad pulsation in the brake pedal. I can check again, but in the past I've verified that it's from the rear brakes.

I'm wondering if it's possible that I have a bent wheel which is somehow pulling the drums into a weird position.

Anyway, I've tried to fix this for so damn long. I've gone through two sets of new drums. The shoes are relatively new, as is the hardware (though I have trouble seeing why this would matter).

I have checked and checked again the hubs for imperfections that could cause the drums to not sit flat. At one point I thought that the studs were not allowing the drum to seat flush against the hub, and I used a grinding bit in my drill press to open the inside of the stud holes on the drum.

I'm ready to get to the bottom of this. I'm thinking I should try rotating wheels, just to see if that makes any difference. And maybe I'll try putting some plastigauge between the drums and the hub to see if they do after all sit flush.

Suggestions are tremendously welcome.

Tue Jan 16, 2018 1:45 pm

I'd try rotating wheels front to back as an easy first step.

Tue Jan 16, 2018 1:59 pm

I've found that the drums can end up very slightly off centre if I don't torque them up while still in the air. Makes for a long, slow pulse in the pedal.

YMMV

Tue Jan 23, 2018 2:40 am

I rotated the wheels front to rear -- there's a big improvement. I somehow never thought to do this.

There is still a slight pulsation, but I haven't verified that it's coming from the back. It's minor enough that I can live with it.

I was convinced at one point that something was interfering with contact between the drum and hub (as I mentioned above). It feels like I can wobble the drum a bit back and forth a bit. So when I had the wheels off and was looking over the brakes, I taped a couple pieces of plastigauge to the inside of the drum, where it contacts the hub, then put the wheel on and removed it again. The plastigauge seemed to show that the drum was sitting flat against the hub.

So, maybe I have one or more bent wheels?