: Anyone convert their 80-81 to rear discs?



jayoldschool
01-19-09, 11:09 AM
The 80-81 RWD cars were the last to use the 8.75" ring gear "P" axle. They came with big 12" drum brakes. Interestingly, this same axle was used in the 77-79 Fleetwood Brougham, and it came stock with rear disc brakes. Has anyone used the 77-79 brakes on the 80-81?

A few questions come to mind:
1. Do the 77-79 components (backing plates, caliper mounts, calipers, rotors) work on the 80/81 axle tube mounts?
2. Does the complete 77-79 axle swap in? Same driveshaft flange?

I am aware that some have used the 94-96 B axle from the SS or 9C1 with rear discs. This can bolt into the D body, but requires the pinion yoke to swapped for the D body flange for the driveshaft. I would prefer to stay with the D body 8.75 axle, and use factory parts to convert to discs.

Anyone have any experience/ideas/thoughts? Or, some 77-79 parts for me:thumbsup:

sven914
01-19-09, 07:26 PM
I'm not sure, but I believe the 76-79 Fleetwood Brougham was equipped with a THM200 transmission (as should yours be), and the wheel base was 121.5 (yours is 121.4), so the parts should be interchangeable.

Aron9000
01-19-09, 07:39 PM
I'm not sure, but I believe the 76-79 Fleetwood Brougham was equipped with a THM200 transmission (as should yours be), and the wheel base was 121.5 (yours is 121.4), so the parts should be interchangeable.

77-79 would be compatible with his year. 1976 was the last year for the really big body, before they shrunk them down.

jayoldschool
01-19-09, 08:34 PM
I'm not sure, but I believe the 76-79 Fleetwood Brougham was equipped with a THM200 transmission (as should yours be),

Nope, they have the big TH400, as does my 81 (which is the last year for it, along with the last year for the Cadillac big block).

Now, any thoughts on the brakes? :D

cadillac_al
01-20-09, 12:08 PM
I believe you are correct that the 77-79 Fleetwood brakes would bolt on your axle. There are also several aftermarket companies making disc brake conversions for just about any old GM car. I don't recall any prices right now but I think they are getting cheaper.

jayoldschool
01-20-09, 12:52 PM
scarebird.com is the best place for rear disc conversions. Their brackets are only about $125 for most applications, then you use factory calipers and rotors to complete the install. They don't list an application for the 77-81 "P" (Cadillac) axle, however. I'll send them an email.

I guess I will get scrounging the yards in the spring. Probably the best bet is to grab a complete axle from a 77-79 Brougham, then rob it for parts to convert mine. If the parts don't fit, I can rebuild the complete axle and swap it in.

472
01-22-09, 03:00 AM
You can easily change to disc by using pre-1980 SeVille brackets (can change nearly any GM axle this way - I did it on my '83 DeVille's 8.875" rear, and my '69 Nova's 8.5").

Cad rear;
http://web.pdx.edu/%7Ebtc/83/images/rdisc05.jpg

The 77-79 axle will swap in FYI.

Attachment is Nova rear;

jayoldschool
01-22-09, 11:20 AM
You can easily change to disc by using pre-1980 SeVille brackets (can change nearly any GM axle this way - I did it on my '83 DeVille's 8.875" rear, and my '69 Nova's 8.5").

Great info, thanks! I will add the Seville to my scrounging. Hmmm, I will have to get two sets so I can do my 71 Pontiac (8.5" Chev 10 bolt) too! Are the brackets available new/repro anywhere, or strictly junkyard? Do you know if they are the same parts as on the 77-79 Brougham?

A few tech questions:
1. Any machining? (eg: turn down the axle flange to fit inside the rotor hat)
2. Use the Seville rotors and calipers?
3. E-brake hook up? Any mods?

472
01-27-09, 12:32 AM
Do you know if they are the same parts as on the 77-79 Brougham?

A few tech questions:
1. Any machining? (eg: turn down the axle flange to fit inside the rotor hat)
2. Use the Seville rotors and calipers?
3. E-brake hook up? Any mods?I'm not sure about stock disc 77-79 cars - never seen one...

1. Yes, actually - that's exactly what needed to happen (only the Cad rear though).

2. I used SeVille rotors on the Cad, 4WD S10 front rotors for the Chev pattern. For calipers I used front calipers, which needs to change. They are too large requiring too much pressure (fluid) to work (mushy pedal feel always), so I will be going with a smaller rear caliper in the future. If you choose to use the bigger ones, be sure the caliper moves freely, I had to file a tiny bit to let them move, and use two passenger side fronts - this keeps the bleeder right side up (notice the SeVille setup puts one caliper in front of the axle, and one behind it). I installed a C3 Vette master cylinder and Wilwood adjustable proportioning valve to compensate, but still not thrilled with the pedal feel.

3. By using front calipers the e-brake was omitted.

jayoldschool
05-17-09, 10:50 PM
Found some more info:

GM had six different rear disc-brake caliper setups from the late 1970s to early 1980s. Most were for Cadillacs, but some were also used by Buick, Pontiac and Olds.

1977-1978 Seville rotors were 5 x 5 inches on an 11 3/16-inch rotor. Axles were 1.4-inch C-clip type.

The 1977 and 1978 De Ville, Riviera and Fleetwood used different calipers and mounting brackets, but the axles and rotors were the same as the above.

The 1979 Seville and the 1979-1981 Firebird with the WS-6 performance package used a similar configuration to the assemblies listed above, although they used a slightly different caliper. The Seville rotors had the 5 X 5-inch pattern and 11 1/2-inch diameter, whereas the Firebird rotors were a 5 x 4 3/4-inch pattern, but had the same outside diameter. Camaros did not offer the WS-6 package on any of their models, although it will bolt on directly to 1970-1981 Camaros. This is the most popular of the GM disc brake swaps, as it fits all 10-bolt and 12-bolt rear ends, as well as the S-10 trucks and Blazers. All of these rears used the four-bolt axle flange measuring 3 1/8 inches across the top and 2 5/8 inches across the bottom. Pay special attention when performing this exchange on the GM 12-bolt axles, because the Seville rotors will not fit without having to machine the center hub of the rotor to fit the axle. The Pontiac WS-6 rotors will not have to be machined to fit the axle.

All 1979 Fleetwood, De Ville and full-size Buicks also used the same axles and rotors as the 1979 Seville; however, the calipers are again different (as well as the caliper mounting bracket) from the above three.

1976-1978 Eldorado rotors were 5 x 5 inches on an 11-1/8-inch rotor. Axle used tapered inner and outer bearings and cups, and 1 3/8-inch and 3/4-inch tapered axle shaft similar in design to a front spindle.

1979-1985 Eldorado, Toronado, and Riviera; and 1980-1985 Seville rotors were 5 x 4 3/4-inch on a 10 1/2-inch rotor. Rear axles were plain steel beam, using a bearing hub assembly bolted onto the end in front-wheel-drive applications.

Because the GM corporate axles are held in by C-clips, it is not necessary to swap these out. However, the axles will have to be removed, both from the donor car and your vehicle, to both remove and install the brackets and backing plates.

One thing you may discover when searching for these parts on GM vehicles is that the left caliper is mounted to the front of the axle, and the passenger-side caliper is mounted to the rear of the axle. This was common on the earliest GM configurations, and does not mean someone has already done a changeover on that vehicle.

jayoldschool
08-05-09, 06:45 PM
Scrounging has finally paid off! Lookee what I just brought home:

http://i40.photobucket.com/albums/e202/jayoldschool/Cadrearend.jpg

It's a complete 1977 Seville disc brake rear end. Of course, this won't bolt into my 81 (the Seville used leaf springs), but I believe (from my research above, and the advice from 472) that the parts will work! So, I will tear it down for the backing plates and calipers. I'll use new rotors, and probably use the calipers for cores (apparently the core charge is really expensive, so I'm glad I got calipers). These calipers have parking brake feature built in, so it will be nice to retain that after the swap. Once I have everything apart I will be able to confirm that the backing plates will work on the big Cad axle (if not, I'll just re-drill them!).

Best part about this (besides having factory Cadillac disc brakes showing through my wire wheels)? I paid 40 bucks for the complete axle :D

greencadillacmatt
08-06-09, 02:01 AM
Am I correct in saying that you don't have to use a proportioning valve when you install disk rear brakes?:confused:

jayoldschool
08-06-09, 11:36 AM
No, you still need a proportioning valve.

I am considering using the master cylinder from the 77-79 FWB that had factory rear disc brakes. It is different than the MC that came on the drum brake cars. We'll see what the pedal feel/effort is like after I get it completed. The good thing is that I have the correct rear calipers. People that have used front calipers on the rear report mushy pedal feel due to the volume of the caliper being too high (and they wouldn't have a parking brake).

csbuckn
08-07-09, 02:21 AM
nice find
is the parking brake gonna bolt up or will it be changed?

jayoldschool
08-07-09, 03:25 PM
Parking brake will bolt up (may need mods, of course...), and must be used.

I have found out that the reason people used to have problems with the Cadillac rear discs is because they DIDN'T use the parking brake. These calipers require the use of the parking brake every time the car is stopped. This causes the piston to ratchet the pads down and maintain contact with the rotor, and when released, the piston stays extended. This keeps the piston out as the pads wear. If you don't use the parking brake, the brakes will be soft, and will require an extra pump to build pressure. I have also found all the info on how to rebuild them, so I will do that myself instead of trading them for rebuilds.

cadillac_al
08-10-09, 11:01 AM
Poor design. Who the hell uses a parking brake anyway? The guy I bought my 77 Fleetwood from said he had to have the rear disc brakes serviced about every 2 years. I haven't noticed any problems with my 78 Seville but I may try the parking brake when I go for inspection. I have a phobia of using parking brakes because they always stick and won't release.

PS damn nice score on that Sevelle rear axle. They make great street rod rear ends.

jayoldschool
08-10-09, 01:25 PM
It would make sense that he would have to have them serviced if he didn't use the parking brake. You should be using yours, too! It's not a poor design... it is uninformed owners (and that is probably GM's fault!). Think about it... they gave us this beautiful system, it self-adjusts, and even releases automatically when the car is put in gear... but we don't use it. Old habits die hard!

I have found some more info that makes it even better. These calipers are specifically designed to use the same residual line pressure as stock drum brakes. Most rear discs require lower residual pressure. This calls for a master cylinder swap, and a different proportioning valve. This is not the case with the factory Cadillac calipers.

jayoldschool
08-10-09, 01:25 PM
It would make sense that he would have to have them serviced if he didn't use the parking brake. You should be using yours, too! It's not a poor design... it is uninformed owners (and that is probably GM's fault!). Think about it... they gave us this beautiful system, it self-adjusts, and even releases automatically when the car is put in gear... but we don't use it. Old habits die hard!

I have found some more info that makes it even better. These calipers are specifically designed to use the same residual line pressure as stock drum brakes. Most rear discs require lower residual pressure. This calls for a master cylinder swap, and a different proportioning valve. This is not the case with the factory Cadillac calipers.

BRICKS
10-24-09, 12:32 AM
didnt think id switch to rear disc because the chances of finding 3.08 limo or brougham seems RARE! but ill be checking for the 3.08 seville rear now. thanks for the info.

drmenard
10-24-09, 12:56 AM
When I found a 78 deville with a posi , it also had disc brakes... this had 2.28 gears...At the time I did not want to take the time to put the discs on .. All the drums bolted right on....The limo does have 3.08 gears , but they have 31 spline axels , not 28 spline, and the drums and brakes are bigger.. and the smaller brakes won't bolt on...

BRICKS
10-24-09, 01:43 AM
hopefully i will have some luck myself this weekend at the yard.
on a slightly similar note, anyone have a spare set of rear sway bar hardware laying around? need everything but the sway bar. apparently the previous owner deemed it useless...?

blacksaphire
02-06-10, 11:52 PM
I know this may be a long shot, but i have a 91 Brougham that I want to convert to rear disk brakes from an SS or LC6. Can someone guild me to post about the convertion

N0DIH
02-07-10, 12:13 AM
I have the 77-79 backing plates and calipers in the garage. Need any measurements?

Also I have a 95 cop car rear axle complete with discs. This is a bolt-in in place of the 7.5", 8.5" and 8.875" axles in the 77-96 B/D Body cars. 3.08 open gears.

Jay, shame you don't live closer!

YourMainParadox
07-10-10, 03:07 PM
Would this be the same for a 74 fleetwood? Also I was told that hearses might have a different axle than a regular fleetwood... is there an easy way to tell? I wish we had junkyards here with classic cars like the one you found that $40 deal on... but I don't really want to set the parking brake every time lol.

N0DIH
07-10-10, 03:31 PM
I have the parts, but don't know if they will work on the 74 axle.

Need to get picts of it to see which axle it is. There is several that would FIT in there, not 100% what axle was used.

So it comes down to tear the backing plates and measure the bolts holes and spacings.

Chevy used the 8.5" that is commonplace today
Buick likely used the same axle as Pontiac and Olds did, the "P" axle, the Pontiac designed 8 7/8" 10 bolt
Cadillac had a funky one that only Cadillac had.

I highly suspect that you can use the 8.5", which would give you the most gear selections for it. As well as likely have a much easier time going with discs....



Would this be the same for a 74 fleetwood? Also I was told that hearses might have a different axle than a regular fleetwood... is there an easy way to tell? I wish we had junkyards here with classic cars like the one you found that $40 deal on... but I don't really want to set the parking brake every time lol.

chp350
07-12-10, 01:19 AM
Would this be the same for a 74 fleetwood? Also I was told that hearses might have a different axle than a regular fleetwood... is there an easy way to tell? I wish we had junkyards here with classic cars like the one you found that $40 deal on... but I don't really want to set the parking brake every time lol.

YMP, I'm confused here on the year of your car. We have exchanged several emails on me helping you locate a hood for your 1973 cadillac and here you are saying a 1974..which one is it ,so I can locate the right hood.
In jacksonville you have a u pull it on normandy that has rear ends for 59.00 dollars, not quite 40 but close enough...lol

YourMainParadox
07-12-10, 07:22 PM
I am pretty sure it is a 74 but I am not completely sure as I have still yet to get the title and the owner said it was a 73 but his dad which knows cars says it is a 74 but the paperwork in the car has 72 and 74 on the dates haha. Either way a 73 hood is the same as a 74 right?


Also I have yet to find any 70s fleetwoods there in the you pull it on normandy.. they are all 80s and 90s

chp350
07-12-10, 07:53 PM
no, hoods are different I believe. check the 6th digit of your vin number. 2 =1972,3=1973,and 4 would be a 1974..from memory 71 to 73 would be the same..
I go every week and last time I saw 2 1976 fleetwoods, and a 1970 which I bought parts off of.

74,75 and 76 use the same tail lights and i can surley say your car is not one of these, hate to say it but Dad is wrong

Bro-Ham
07-12-10, 08:09 PM
post a picture of this car so we can identify it. :)

YourMainParadox
07-12-10, 08:51 PM
http://img534.imageshack.us/img534/5503/image005nh.jpg

http://img543.imageshack.us/img543/3046/image004x.jpg

http://img41.imageshack.us/img41/6926/1974caddysuperiorhearse.jpg

http://img690.imageshack.us/img690/6926/1974caddysuperiorhearse.jpg

http://img97.imageshack.us/img97/6926/1974caddysuperiorhearse.jpg

http://img684.imageshack.us/img684/7263/img9935medium.jpg

csbuckn
07-13-10, 12:53 AM
Wow...what a car.

Bro-Ham
07-13-10, 12:54 AM
That's a 1973. :)

csbuckn
07-13-10, 12:57 AM
How can you tell? Is it the headlights?

Bro-Ham
07-13-10, 01:17 AM
How can you tell? Is it the headlights?

cs, Cadillac always changed the cars in some way every year until the mid/late 1980's. That hearse has a 1973 front end. The front and rear of the 1974 is different, the rear of the 1974 looks like the 1975 and 1976.

YourMainParadox
07-13-10, 08:32 AM
What is the difference between the 73 and 74 then? Also what years will the hoods swap?

Oh and we are way off the original topic here <.<

Bro-Ham
07-13-10, 10:07 AM
What is the difference between the 73 and 74 then? Also what years will the hoods swap?

Oh and we are way off the original topic here <.<

I agree, you should start a new thread so this one stays true to disc brakes. Just do some google image searching and you should be able to see the differences between the two years. I doubt the hoods will be interchangeable between 73 and 74.