G-Body rear disc brakes

G-Body rear disc brakes

READ THIS ENTIRE POST BEFORE BUYING ANY PARTS!

There seem to be lots of people wanting to do disc brakes on the back end of their G-Body. There are kits from various vendors like Baer and Wilwood and Aerospace Brakes, but those kits are REALLY pricey. They’re likely worth it, but we’ll talk more about that later.

What we’re going to talk about here is attempting to do the conversion more cheaply. Lots have done it, but nobody that I’ve been able to find has really documented it well.

The set up I’m going to install here is based on Speedway Motor’s kit. It uses the rear calipers and rotors from a 1995 Cadillac El Dorado, along with some new backing/caliper mount plates from Scarebird Brakes. I chose to piece the kit together myself instead of buying Speedway’s because I think the Scarebird bracket is better. It’s one piece where the Speedway bracket is two, and it has a hose lock mount, which makes the install much cleaner. Also, the Speedway kit doesn’t include some necessary stuff, like flexible lines to adapt the axle hard line to the caliper. This is no small detail, and Speedway doesn’t even put the length and ends required in the instructions.

New, before we get into the nitty gritty, I’m going to answer two questions that I was never able to find answers for.

1. Do disc brakes save weight compared to drums?

No.

At least, not with this setup. The El Dorado calipers are iron, and the rotor is not a flimsy piece. I weighed what I took off and what I put on, and this disc setup is five pounds heavier than my steel drum setup. This is where the high-end kits from Aerospace and Baer shine: They use aluminum calipers to save several pounds.

2. If it’s not lighter, why bother?

Because physics. Disc brakes are able to shed heat much faster than drums, so if you’re going to do any kind of road course work with the car, you want disc brakes. The drums work fine for autocross, but I actually had a problem at the last Wilmington Champ tour from heat buildup inside the drums. That course was nearly a mile long, and at the end of my last run, I had a little fade. Back in the paddock, after the car had sat while I changed tires, the shoes stuck to the drums and the car wouldn’t move until I judiciously applied some throttle to break them back loose.

3. Is a disc brake conversion a bolt and go?

Not on this platform.

At least, with this particular setup, you must have the axle flanges (the part with the lug studs in it) turned down by 0.190″. Otherwise, the rotor won’t fit over the flange. The Scarebird bracket eliminates any cutting or drilling on your axle housing. Other kits require enlarging the holes in the housing flange, and some even require cutting off the housing flanges and welding on new ones with bigger bearings and a bigger flange surface. Do your homework here, or you could end up with a brake kit you can’t install.

This route, in my opinion, is the easiest. It’s nearly a bolt in, and the parts are common GM stuff. New pads, rotors, and calipers are only as far away as your local parts store.

So, on to the parts list for attempt one:

Part Number Quantity Description Price Notes
NA 1 Pair of Scarebird Brackets $124 Scarebird Brakes
AAZ-18-4138 1 Caliper Passenger side $76.97 Summit
AAZ-18-4139 1 Caliper Driver Side $76.97 Summit
AXT-AXCD154 1 Bendix Ceramic brake pads $18.97 Summit
BEN-PRT1261 2 Bendix Rotor $71.88 Summit
RUS-657300 2 Brake Hose $47.97 Summit
WIL-260-13783 1 2LB residual pressure valve $19.78 Summit
Parts Total $436.54

Some other odds and ends you’ll also need: Gear oil  and brake fluid. I already had some, so I didn’t have to buy more. If you’re looking at the Speedway kit, this route is cheaper by about seventeen bucks after you’ve added the brake lines and the residual pressure valve.

The brake lines are Russel braided stainless units, 9″ long with a 3/8-24 inverted flare on one side, and 10mm banjo on the other.

The  two pound residual pressure valve is important. It keeps 2psi of pressure in the rear line, which keeps the pads pressed up against the rotor and help prevent knockback. It replaces the 10psi residual pressure valve you should have with your drum brakes (see my other posts from when I replaced my master cylinder).

I just got the $18 ceramic pads. I run Hawk HPS on my front brakes, but the rears don’t handle that much of the load, so I’m not convinced that I needed Hawk product on the back. If they prove to suck, I can always change them later.

Now, on to the how-to.

The first step is obvious, get the car off the ground and safely supported on jack stands, then remove the wheels.

Drum!

Now, get the drum off. Depending on when you last serviced your brakes, this part might be easy, or it might be hard. It was pretty easy for me, they wiggled right off.

Drum gone! Shoes and springs!

With the drum out of the way, you have to take apart the shoe assembly. Take a picture of it before you do, just in case something happens and you have to put the drums back on. Remove the circular clips that hold the shoes on at the bottom, then get the springs unhooked from the pin at the top, and the whole mess will fall off the axle.

Next, pop the differential cover, remove the center pin in the differential carrier, and get the C-clips out, then pull the axles. Now is a good time to inspect your ring and pinion and the spider gears.

Now, the axle flange. I have read many a note regarding having to turn the axle flange down so it will fit into the new disc hats. But nobody says by how much. One of my favorite stories was a guy that put the car up in the air, started the engine, put it in gear, and then held a grinder up against the flange while the engine spun it. He just did that until the rotors went on. Novel, but I like being more precise. I also don’t want metal shavings all over the damn car.

So, I sat one of the drums down on the garage floor with the wheel face side up, then put the new rotor on top of it, and lined up the lug holes. I then stole some play-doh from my daugher’s ample supply, and placed it as such:

Play-doh!

This allowed me to carfully set the axle down into the rotor. The putty compressed where it interfered, and gave me something to measure.

Test fit

Measure that squished part

Measure from the edge of the flange to the edge of that smashed putty, and you know exactly how much to take off the flange. Mine measured anywhere from 0.144″ to 0.160″. I had the machine shop turn it down by 0.200″ to make sure it would fit.

So, for google: You must turn down the axle flange by  at least 0.200 inch to get it to fit inside the rotor hat. More on this later.

While your axles are at the machine shop, go ahead and soak the brake line nut, the giant nut at the top of the backing plate that holds the wheel cylinder, and the two smaller bolts at the bottom in penetrating oil, and let them sit. After the liquid wrench has had time to work, hopefully you can break the line nut loose without destroying the brake line. Then you can remove the wheel cylinder and the backing plate.

Everything removed

Now would be a good time to replace the wheel bearings.

But there’s a catch here. There is a lot of slop in the axles on a C-clip rear end. This causes something called knockback, where the axle assembly moves laterally, with the disk moving along with it. This will knock the rear caliper pistons back into the caliper. The next time you go for the brakes, you’ll have a looooong pedal as the rear calipers push the pads back up against the rotor. Now that you have it apart, it’s an opportune time to take care of this.

I didn’t take care of this when I originally did the swap. More on that later.

From here, it’s easy. You put the new Scarebird caliper plate on. You put the axles back in. You put on the new brake hoses and attach the calipers. Then you mount the rotor and bolt the calipers to the bracket. Easy peasy. You swap the e-brake cables side-to-side to get enough length and hook them up.

All buttoned up? Ebrake isn’t connected yet, but it’s basically there

Then you do what I did: Put the wheels on and go for a test drive. During that test drive, you will probably realize the parking brake mechanism on the calipers hits the frame. That’s what happened to me. Oops. The El Dorado caliper also uses a ratchet mechanism that’s powered by the parking brake to seat the pads against the rotor. I never could get it adjusted properly. The brakes sucked.

so, tear it back apart and go shopping for a new caliper.

I settled on Wilwood’s 120-9333 caliper. It’s a lightweight cast-iron single-piston caliper that uses the same GM D154 pad as the El Dorado calipers. And it has no parking brake provision. That gave me the clearance I needed. I lost the parking brake, though. But I have an automatic transmission. The parking pall is way stronger than any parking brake would be.

But that changes the parts list:

Part Number Quantity Description Price Notes
NA 1 Pair of Scarebird Brackets $124 Scarebird Brakes
120-9333 2 Caliper Passenger side $159.90 Summit
AXT-AXCD154 1 Bendix Ceramic brake pads $18.97 Summit
BEN-PRT1261 2 Bendix Rotor $71.88 Summit
RUS-657300 2 Brake Hose $47.97 Summit
WIL-260-13783 1 2LB residual pressure valve $19.78 Summit
Parts Total $444.50

Here’s where the price started to climb. Sure, $444.50 total for what was on the car, but I had $180 ins useless El-Dorado calipers sitting on the floor. I eventually sold them at a swap meet for $75.

So, put all this together, and they worked!

Until I went to my first event. Knockback. Every corner I had to pump the brakes to push the rear pistons back against the rotor. It sucked. It was really bad in a slaloms. Upon entry, they’d be fine, but after one or two shimmies, I’d go to tap the brakes and there’d be none. It made things difficult.

So, how to fix the knockback? I was already using a floating caliper. I had to stop the axle flange from moving.

After some research, I settled on the Strange A1033 C-Clip eliminator kit. It provides a tapered outer bearing suitable for street and track use and adds a good bit of safety. A broken axle will not result in the wheel separating from the car.

Installation of the C-Clip eliminators is straightforward, but does require cutting on the axles and housing. There’s no going back. I also stepped up to a better brake pad set.

So here’s the new parts list:

Part Number Quantity Description Price Notes
NA 1 Pair of Scarebird Brackets $124 Scarebird Brakes
120-9333 2 Caliper Passenger side $159.90 Summit
A1033  1  Strange C-Clip Eliminator  165.99
DP41146R 1 EBC Bluestuff Pads $127.90 Summit
BEN-PRT1261 2 Bendix Rotor $71.88 Summit
RUS-657300 2 Brake Hose $47.97 Summit
WIL-260-13783 1 2LB residual pressure valve $19.78 Summit
Parts Total $717.42

 

We ended up ~$300 above the initial estimate by the time we were done. Youch. Not such a bargain, especially after factoring in all the time spent working the bugs out, and the money sunk into parts that were tried and removed isn’t in this total.

But, did it work?

Sort of. The C-Clip eliminators definitely make it better. But they don’t remove all the knockback. They just can’t. There’s not enough structure on our housing ends. The whole thing flexes under load. The Eliminator kit has turned out to be a wear item. After a season, I had a considerable amount of lash in the left side, so I replaced the entire kit. The bearings and races and seals added up to nearly $135 and were flung all over the country, which meant extended shipping times. The entire kit was in stock just up the road in Cincinatti. So the extra $30 to get them all in one box and delivered overnight made sense.

Does it work fine on the street? Absolutely. But for serious track duty, you need to bypass this whole mess and go straight to a full-floating axle assembly. There’s just no way around it. Everything else has turned out to be a frustrating band-aid.

If I were to do this all over again, I would have tried a set of Kirban aluminum drums with new shoes shod in the EBC compound, drilled the drum backing plate for ventilation, and built rear brake ducts. The drums seem expensive at $200 a set, but they slip straight on. They shed 10lbs of unsprung rotating mass off each corner, and no knockback problems. They’d be worth a try.

 

 

 

Andrew

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