67 disc brake upgrade - C3 corvette rotors?
 

I've been researching for a drum to disc brake upgrade on my 67 Lemans for years now. I upgraded to 15" rally II wheels a couple years ago in prep for this upgrade, so my requirement is that is must fit with those wheels. I was going to just go with a stock 69-72 style swap, but started to find that I could get better brakes for less money if I plan this out well.

I found a popular swap is 98-up F-body rotors and calipers with drum brake spindles and hubs using a custom mounting bracket. I happened across a 2000 f-body while at the salvage yard a while back, so I grabbed the calipers and rotors for cheap. I knew that there was a high likelihood these would not work with the 15" rally II wheels, and that is the case. The calipers protrude beyond the hub face where the wheel mounts and that is a problem. Diameter wise it does look like they will fit inside the 15" wheel.

I started looking for rotors that would allow me to space the calipers further inboard and came across some discussion around using C3 corvette rotors (11.75" diameter and 1.25" thick). It looks like this was even an option on 68-69 Camaros known as the J52 and/or JL8 brakes? Since the Camaro uses the same spindle it seems like this might be an option. I even cam across some old discussions on this site while searching where it looks like Scarebird was working on a kit that used the C3 rotor. However it was 10 years ago and I don't see such a kit listed on his website.

I'm curious if anyone has done a disc brake swap using the C3 corvette rotor and if you can give me any guidance on doing so? I would like to use the 98-up F-body caliper still if possible since I already have them and they are aluminum monster size PBR calipers (I realize I'll have to make my own custom brackets to do so). Otherwise it looks like an 80's GM truck caliper might be my next best option. Short of tearing my car down and rendering it undriveable until I complete the swap, I'm not sure how to proceed at this point. I feel like I need the measurement from the drum hub face to the spindle brake mounting surface to figure out if I can make this work, but have been unable to find that measurement listed anywhere on the internet. I'm just about to buy a spindle and hub just to mock this up with.

Any advice?

We dropped the C3 rotored setup as it required a custom hub and drove the price above a point where it was competitive.

We did however develop another heavy duty variant that is similar in size but can reuse the stock drum hub and spindle. We use the 90's GM truck caliper and Jaguar rotor to do this.

https://scarebird.com/image/catalog/...mall%20800.jpg

Scarebird, what are the specs on that rotor / kit? I don't see the specs listed on your website, am I missing it? Perhaps a parts list or instructions so I can look up and price out the parts? The C3 rotor is appealing to me because the 11.75" diameter and 1.25" thickness is a significant upgrade over the stock 11" x 1" rotor. The 1.25" thickness also happens to be compatible with the f-body calipers I have.

To use c-3 Vette rotors you need to use disk brake spindles and drum brake wheel hubs with longer studs. I welded up and redrilled my caliper brackets and used the ‘81-‘91 half ton truck calipers. Make sure calipers are for 1.25” rotors as the half ton trucks were available with 1” rotors also.

I have this setup on my Judge right now, stopping power is greatly increased and fade resistance is much better.

Nate

Going to revive my thread as I never did get to upgrading my front brakes to discs. My front drum wheel cylinders are now leaking, so it's time to do something. I've got it tore down to the bare spindles so I could get measurements to figure out what I can and can't do. The 98-up F-body rotors/calipers won't work with the 15" rally 2 wheels (which I'm not willing to change at this time). I thought I might be able to use the F-body PBR calipers with C3 corvette rotors, but it looks like the calipers will hit the frame/suspension when the wheels are turned - so I think that option is out. Looks like there's only 1 option available from Scarebird since the new ownership - and that's for 14" wheels and would be worse than stock 69-72 disc system since it uses smaller rotors to fit 14" wheels. So, I think I'm looking at piecing together a stock 69-72 disc system (11" x 1" thick rotors) for about $400 or coming up with something else homegrown.

Option 1: Would be a stock 69-72 system. Cut the upper boss on my drum spindles to fit the stock disc brake bracket (don’t trust aftermarket Chinese spindles). Then order stock rotors, calipers, hoses, etc from RockAuto for about $400.

Option 2: C3 rotors, with turned down drum hubs (I have a lathe to do this myself), and then use Wilwood Dynalite-M 4 piston calipers (120-17773) with DIY brackets to end up with something better that the stock 69-72 discs for not too much more money. I think this would still fit my 15” wheels. I think I could do this for around $600. I would expect 4 piston calipers with 11.75” x 1.25” thick rotors to be a fairly significant upgrade over the stock 69-72 disc brakes.

Option 3: Replace drum brake wheel cylinders and keep putting off the disc upgrade so I can get my car back on the road without going broke in this current economy.

I’m also thinking about eliminating my power brake booster and just going with manual brakes. I have limited vacuum with my big cammed 469 and think I could get better brake feel with manual brakes.

My only real goal with the brake upgrade is to eliminate/reduce brake fade. The current 4 wheel drums will lock all 4 wheels in a panic stop situation, but they can only do it once! Then I have almost no brakes until they cool off. Reducing weight on the front of the car would be a nice “bonus” with the aluminum Wilwood calipers.

Any other suggestions or known issues with what I am considering?

For posterity sake, incase anyone come along in the future looking into doing what I did... The drum hub to the top mounting boss on the drum spindle measure right around 2.8125 to 2.8135 inches. If I had known that measurement before tearing my car down and ordering parts, I probably would have went with different options. That boss would have cleared the C3 rotor with no modification needed to the spindle. I would have had to make custom brackets that didn't use that mounting point since there isn't enough clearance though.

Instead, I went with the Pozzi Racing cheap big brakes (https://www.pozziracing.com/cheap_big_brakes.htm) style setup.

I ordered cheap stock 68-72 style caliper brackets from ebay since I knew I was going to have to modify them. That was my biggest mistake/regret because they were absolute garbage! The threaded holes for the caliper pins were crooked, they were bent wrong, wouldn't even fit the calipers. I spent hours bending, tweaking, grinding, cutting and welding on them to get them to be useable. I'm still not happy with the results, but I think they are as good as they can get. Would have been super easy to modify as needed if those brackets had been decent quality!

I cut down the upper brake mounting boss on my stock drum spindles to make them the same as disc brake spindles. I think I cut off about .610" off that boss.

I used the stock drum hubs (no modification needed), but did upgrade them to 1/2" studs to match my Mosier 12 bolt rear so that I now have 1/2" studs with the same size lug nuts all the way around. That added about $100 in cost.

Then went with the C3 11.75" x 1.25" rotors and late 70's GM truck calipers with 3" pistons to fit the wider 1.25 thick rotors. I also splurged for braided stainless brake hoses from Inline tube at around $90. All in, I think I ended up spending around $450 total.

Unfortunately, I feel like I lost braking power with this setup. Granted, I haven't fully bedded in the new pads/rotors just yet, but initial feel is not what I was expecting. I was planning to eliminate my power brake booster and go with manual brakes. However, I decided to try it out without making any changes to the booster or master cylinder first. I expected the brakes to be super touchy and just about throw me through the windshield under light braking with the booster still in place. But no, it feels like I lost braking power compared to the stock drum setup. I think the master cylinder is likely a 1" bore since that should be what was original for power drum brakes, although it is an aftermarket replacement master cylinder. I think I'm going to need to go down in master cylinder bore size, but I'm concerned about a smaller bore having enough volume to work with the large caliper pistons. Anyone have experience or suggestions on that?

Thanks,
Lee

I put Wilwood 4 piston brakes on my 67 Firebird. Here's a list of parts I used from Jegs. My master cylinder bore is 1.125 in. Do you have a second hole on your brake lever? If so try putting the linkage in the other hole.

64 Impala/Corvette kit i got uses a bracket to mount a D52 caliper and uses a 79-81 TA REAR 11' rotor. CPP makes an aluminum hub or you can turn down the drum hubs to 6". Migth give you some ideas
2002 Camaro/TAs us a 12" rear rotor that looks close t the same offset if you make a bracket and want a 12" rotor.

As spec'd on a 70's c10, these calipers would likely have been paired with a 1.125" bore master cylinder. These D52 calipers have huge pistons and need a lot of fluid to work.

It seems like you're making assumptions regarding the size of your master cylinder and it also seems like your brake booster is likely the booster found on a power drum brake application?

The power drum brake boosters were usually small and don't offer as much power assist as a booster mated to a power disc car.

It sounds like you need to decide at this point if you want to keep power brakes or go manual. If you decide to keep the power brakes, you likely want the larger 1.125" master cylinder. If you want to go manual, you can work with the 1" master cylinder, but you may want to consider a 15/16" bore master cylinder to help out with leg pressure.

I may be old fashioned but unless you are road racing or auto crossing your car I really can't see why you need bigger than the 11 inch disc brakes. Why do you think you need the expense and complexity of bigger brakes. I race a 3800 lb car and have never had any problem stopping it from in excess of 125 mph with 1971 Cutlass brakes that I added.

Thanks for the feedback everyone!

I finally got a chance to put about 10 miles on the car and do some bedding for the new brakes. They are certainly getting better as they bed in, so maybe not worse after all, but certainly not the night and day difference I was expecting. Once I get some more driving on them, then I can try some hard braking performance testing with some back to back panic stops to see if they are much better or not.

JLMounce - I am making assumptions. I don't know what may or may not have been changed about this car before my ownership (1991). I also don't necessarily remember what all I have done to this car in the 34 years I have owned it (I'm getting too old to remember things anymore). I do know that when I first got the car, I re-did most of the brake system with what was likely whatever AutoZone or the like sold me back then. Should have been stock replacements - but who knows. This car has always been power drum brakes up until now, so I again would assume the booster is for power drum brakes - I didn't even know there was a difference in the boosters. The one on the car barely clears the valve cover, which is another part of why I was planning to change to manual brakes and eliminate the booster - it seems pretty big to me.

I fully intended to go manual brakes, but now I'm not so sure? If what I have now is power assisted 1" bore master, then I believe going to a larger bore master cylinder (like a 1.125") is going to make the brakes worse - with more pedal effort required? I also believe eliminating the booster and staying with a 1" bore master is also going to make the pedal effort higher? That's why I'm not so sure what I should do now. If I understand correctly, the 70's trucks were available with manual brakes and those same D52 calipers - and they used a 1" bore master. That's why I thought my existing master would be OK. That does make me want to try going to maybe a 15/16" or 7/8" bore master, but I'm not sure if that small of bore can supply the fluid volume required.

Goatracer1 – I didn’t have the 11” disc brakes, this car was 4 wheel drum. I went with the bigger stuff because it was less expensive than going with the stock style 11” disc system. I just got done dumping $5000 into the built rearend when the front drum brakes decided to start leaking, so I needed to save some money if possible while still getting my car drivable again. And those drum brakes had to go!

Regardless of whether or not the system is manual or boosted, as the master cylinder bore gets larger, the pedal travel reduces, while the pedal effort increases. As the bore gets smaller, pedal effort decreases, while pedal travel increases.

It's a dance to get it right. However, boosted systems since they are helping you can benefit from larger bore master cylinders since they are helping you out. If you're finding that the pedal effort is easy but that the car isn't stopping until you get the pedal near the end of its physical travel, going to the larger bore master cylinder will decrease the amount of pedal travel necessary to get the brakes working. You may not even notice a change in the pedal effort. If you do notice, it's still a boosted system, you're still going to be using your foot and ankle, not your leg.

I dealt with many of the issues you are currently dealing with. Lack of real stopping power, long/soft pedals. I tried a bunch of different combinations and after ending up with a dual 9" booster with a 1.125" bore, the brakes worked really well, but there was so little pedal travel at the amount of boost provided that they ended up being very on or off, with very little granularity and no feel whatsoever. I was tired of trying different combinations, bleeding brakes over and over and wasting a bunch of money on parts.

I finally threw in the towel, got on the phone with Wilwood and had them spec me a manual system from the ground up. I should have done that in the first place. With the brakes I have on my bird now, I'm a believer in manual brakes. The pedal is firm and direct, but offers great modulation and feel. While pedal effort requires your leg as well as your ankle and foot, it's not a bad firm. Not like with a power system that has lost its boost and becomes rock hard with no braking power. The pedal firm with my manual set up is firm, but brakes very linearly with excellent stopping power.

With the D52 calipers, I would not try a 7/8 bore master. While that would certainly reduce pedal effort, it may not be enough fluid volume to move the calipers. I would go with the 15/16" bore master, or you can try your 1" bore first. The 15/16" bore is only 6.25% smaller. Theoretically speaking it should offer about that much reduction in force necessary while requiring that much more pedal travel. For a street car in stop and go traffic, that 6% or so less pedal effort could be welcome.

It's all in what you want from the system, which is kind of the glory of manual brakes in general. If you want soft with lots of pedal travel for modulation, you can tune that in. If you don't mind some leg effort and what your brake travel very minimal, you can dial that in as well. Just change the master cylinder size for what you want.

Here's a fun observation, guys that run manual brakes in auto-x cars, sometimes run the larger bore master cylinders. Especially if they don't left foot brake. By reducing the pedal travel, they can get their braking done with less foot movement, and get back to the gas quicker.

To many assumptions here for decisions concerning brake systems.

For starters you need to measure your current fluid pressure at the caliper to determine if there is a problem with your master cylinder. You could have pitting in the bore that is bypassing fluid and preventing it from generating full pressure.

https://www.summitracing.com/parts/s...-hSQLBvuKgG4eE

Then take 15 minutes to remove the master cylinder and measure the master cylinder bore.

Then measure the brake pedal ratio. There are different ratios for manual and power brake cars.

https://shop.wilwood.com/blogs/news/...w-to-find-them

With this information you can run the numbers and know what pressure to expect at the caliper.

To go to manual brakes you will need to have the correct pedal ratio.

You haven't mentioned if you using the original distribution block or has it been replaced with a later model disc brake proportioning valve? Either one will work but if it ìs the original drum brake distribution valve that will cause the rear brakes lock up on hard stops unless you put an adjustable proportioning valve in the rear brake circuit.

https://www.jegs.com/i/JEGS/555/6302...CABEgJruvD_BwE

1971 Firebirds could be ordered with manual disc brakes from the factory. They used the D52 calipers. If you have access to older parts books you can determine which master cylinder they used. I used to know off the top of my head but that bit of information seems to have disappeared. I used to have one and those brakes worked well.

My 67 Suburban has D52 calipers with a 1" bore master cylinder and it stops well and it weights 5000 lbs. One of the best changes I made after going to discs was buying the best pad for my application. Not cheap (Compared to parts store pads)but, much cheaper than replacing my 15" wheels and going to larger rotors and multiple piston calipers. You will need to converse with a quality vendor like Hawk or Wilwood to get the correct pads for your usage and application.

I pulled the booster out last night thinking I might be able to use my existing master cylinder for a manual setup, but didn't work out. The pedal linkage rod is not easily removable from the booster so I'll have to order a manual pedal linkage rod before I can try that. I measured the back of the master cylinder with calipers while I had it apart, and got around 1.065" which seems odd. I don't think GM had a 1-1/16" bore option. I'm still thinking it's a 1" bore and there's just a little larger opening at the area I can measure.

Keep in mind, I've been researching and planning for this brake upgrade for over 10 years. I've done a lot of reading and learning while trying to plan something out that I could afford. I'm not some completely ignorant fool who just wanted random people on the internet to do all the research for me and tell me what to do. I've talked to most of the "brake suppliers" over that time and have not been overly impressed by their supposed professional advice.

The pedal ratio is the same as all GM A-body's of that era, about 3.5:1 for the power brake hole and about 6:1 for the manual brake hole. It's currently in the power brake hole because I have power brakes and because the booster is at an angle that only allows connection to the power brake position. I will use the 6:1 manual position if/when I go to a manual setup.

I'm using the stock drum/drum distribution block (for now). I thought I'd try it that way before adding an adjustable proportioning valve to the rear brakes based on feedback I read on the internet indicating that it typically ended up adjusted to full open anyway. So far I have not encountered rear lock up with it the way it is.

I have good high quality new pads that are either GG rated or GH rated - I can't remember now and I forgot to take a picture for reference.

I do plan to eventually replace the master cylinder with a new one, because then my entire brake system will be new. I wanted to get a point of reference from what I have now to determine what to go with. Right now I'm leaning toward trying to find a suitable 1" or 15/16" bore manual master cylinder - preferably in aluminum and much less than Wilwood's overpriced $300 units.

That sounds like your issue: the bore is too small for power disc.

I do not know if the boosters (drum, disc) are interchangeable as the stroke may be different. I do know that the 1974-80 1/2 ton truck manual will work perfectly with a 2-15/16" bore caliper (such as D52 pads) as I have one on my Lemans. You will need to mod the pushrod to fit your pedal arm and the reservoirs are switched front to rear.

https://www.rockauto.com/en/catalog/...+cylinder,1836

https://www.rockauto.com/en/moreinfo...t=1836&jsn=713

1.06 is about the measurement I've gotten when measuring a one inch master cylinder bore in the past.